CIDOC-CRM version: 7.1.1

Produced by the CIDOC CRM

Special Interest Group

Version 7.1.1

April 2021

Editors: Chryssoula Bekiari, George Bruseker, Martin Doerr,

Christian-Emil Ore, Stephen Stead, Athanasios Velios

CC BY 4.0

2021 Individual Contributors to CIDOC CRM 7.1.1

https://doi.org/10.26225/FDZH-X261

This page is left blank on purpose

This document is the formal definition of the CIDOC Conceptual Reference Model (“CIDOC CRM”), a formal ontology intended to facilitate the integration, mediation and interchange of heterogeneous cultural heritage information and similar information from other domains, as further detailed below. The CRM is the culmination of more than two decades of standards development work by the International Committee for Documentation (CIDOC) of the International Council of Museums (ICOM). Work on the CRM itself began in 1996 under the auspices of the ICOM-CIDOC Documentation Standards Working Group. Since 2000, development of the CRM has been officially delegated by ICOM-CIDOC to the CIDOC CRM Special Interest Group (SIG). The SIG, in turn, collaborates with the ISO working group ISO/TC46/SC4/WG9 to bring the CRM to the form and status of an International Standard. This set of collaborations has resulted in the production of ISO21127:2004 and ISO21127:2014, the ISO standard editions of the CIDOC CRM. This collaboration will be continued in order to support the next update of the ISO standard edition. The present document belongs to the series of evolving versions of the formal definition of the CRM, which serve the ISO working group as community draft for the standard. Eventual minor differences, in semantics and notation, of the ISO standard text from the present, community CIDOC CRM version, which the ISO working group requires and implements, will be harmonized in the subsequent versions of the present, community CIDOC CRM formal definition document.

The primary role of the CIDOC CRM is to enable the exchange and integration of information from heterogeneous sources for the reconstruction and interpretation of the past at a human scale, based on all kinds of material evidence, including texts, audio-visual material and oral tradition. It starts from, but is not limited to, the needs of museum documentation and research based on museum holdings. It aims at providing the semantic definitions and clarifications needed to transform disparate, localised information sources into a coherent global resource, be it within a larger institution, in intranets or on the Internet, and to make it available for scholarly interpretation and scientific evaluation. These goals determine the constructs and level of detail of the CIDOC CRM.

More specifically, it defines, in terms of a formal ontology, the underlying semantics of database schemata and structured documents used in the documentation of cultural heritage and scientific activities. In particular it defines the semantics related to the study of the past and current state of our world, as it is characteristic for museums, but also or other cultural heritage institutions and disciplines. It does not define any of the terminology appearing typically as data in the respective data structures; it foresees, however, the characteristic relationships for its use. It does not aim at proposing what cultural heritage institutions should document. Rather, it explains the logic of what they actually currently document, and thereby enables semantic interoperability.

The CIDOC CRM intends, moreover, to provide a model of the intellectual structure of the respective kinds of mentioned documentation in logical terms. As such, it has not been optimised for implementation specific storage and processing factors. Actual system implementations may lead to solutions where elements and links between relevant elements of our conceptualizations are no longer explicit in a database or other structured storage system. For instance, the birth event that connects elements such as father, mother, birth date, birth place may not appear in the database, in order to save storage space or response time of the system. The CIDOC CRM provides a conceptual and technical means to explain how such apparently disparate entities are semantically and logically interconnected, and how the ability of the database to answer certain intellectual questions is affected by the omission of such elements and links.

The CIDOC CRM aims to support the following specific functionalities:

Inform developers of information systems as a guide to good practice in conceptual modelling, in order to effectively structure and relate information assets of cultural documentation.

• Serve as a common language for domain experts and IT developers to formulate requirements and to agree on system functionalities with respect to the correct handling of cultural contents.
• To serve as a formal language for the identification of common information contents in different data formats; in particular, to support the implementation of automatic data transformation algorithms from local to global data structures without loss of meaning. The latter being useful for data exchange, data migration from legacy systems, data information integration and mediation of heterogeneous sources.
• To support associative queries against integrated resources by providing a global model of the basic classes and their associations to formulate such queries.
• It is further believed that advanced natural language algorithms and case-specific heuristics can take significant advantage of the CIDOC CRM to resolve free text information into a formal logical form, if that is regarded beneficial. The CIDOC CRM is not thought, however, to be a means to replace scholarly text, rich in meaning, by logical forms, but only a means to identify related data.

Users of the CIDOC CRM should be aware that the definition of data entry systems requires support of community-specific terminology, guidance to what should be documented and in which sequence, and application-specific consistency controls. The CIDOC CRM does not provide such notions.

By its very structure and formalism, the CIDOC CRM is extensible and users are encouraged to create extensions for the needs of more specialized communities and applications.

The overall scope of the CIDOC CRM can be summarised in simple terms as the curated, factual knowledge about the past at a human scale.

However, a more detailed and useful definition can be articulated by defining both the Intended Scope, a broad and maximally-inclusive definition of general application principles, and the Practical Scope, which is expressed by the overall scope of a growing reference set of specific, identifiable documentation standards and practices that the CIDOC CRM aims to semantically describe, restricted, always, in its details to the limitations of the Intended Scope.

The reasons for this distinctions between Intended and Practical Scope are twofold. Firstly, the CIDOC CRM is developed in a “bottom-up” manner, starting from well-understood, actually and widely used concepts of domain experts, which are disambiguated and gradually generalized as more forms of encoding are encountered. This aims to avoid the misadaptations and vagueness that can sometimes be found in introspection-driven attempts to find overarching concepts for such a wide scope, and provides stability to the generalizations found. Secondly, it is a means to identify and keep a focus on the concepts most needed by the communities working in the scope of the CIDOC CRM and to maintain a well-defined agenda for its evolution.

The Intended Scope of the CIDOC CRM may, therefore, be defined as all information required for the exchange and integration of heterogeneous scientific and scholarly documentation about the past at a human scale and the available documented and empirical evidence for this. This definition requires further elaboration:

• The term “scientific and scholarly documentation” is intended to convey the requirement that the depth and quality of descriptive information that can be handled by the CIDOC CRM should be sufficient for serious academic research. This does not mean that information intended for presentation to members of the general public is excluded, but rather that the CRM is intended to provide the level of detail and precision expected and required by museum professionals and researchers in the field.
• As “available documented and material evidence” are regarded all types of material collected and displayed by museums and related institutions, as defined by ICOM^[1], and other collections, in-situ objects, sites, monuments and intangible heritage relating to fields such as social history, ethnography, archaeology, fine and applied arts, natural history, history of sciences and technology.
• The concept “documentation” includes the detailed description of individual items, in situ or within collections, groups of items and collections as a whole, as well as practices of intangible heritage. It pertains to their current state as well as to information about their past. The CIDOC CRM is specifically intended to cover contextual information: the historical, geographical and theoretical background that gives cultural heritage collections much of their cultural significance and value.
• The documentation of collections includes the detailed description of individual items within collections, groups of items and collections as a whole. The CIDOC CRM is specifically intended to cover contextual information: the historical, geographical and theoretical background that gives museum collections much of their cultural significance and value.
• The exchange of relevant information with libraries and archives, and the harmonisation of the CIDOC CRM with their models, falls within the Intended Scope of the CIDOC CRM.
• Information required solely for the administration and management of cultural institutions, such as information relating to personnel, accounting, and visitor statistics, falls outside the Intended Scope of the CIDOC CRM.

The Practical Scope^[2] of the CIDOC CRM is expressed in terms of the set of reference standards and de facto standards for documenting factual knowledge that have been used to guide and validate the CIDOC CRM’s development and its further evolution. The CRM covers the same domain of discourse as the union of these reference standards; this means that for data correctly encoded according to these documentation formats there can be a CIDOC CRM-compatible expression that conveys the same meaning.

As part of the on-going work of keep the standard up-to-date, discussions on the types of bias present in the CIDOC CRM are in progress within the CIDOC CRM community.

Users intending to take advantage of the semantic interoperability offered by the CIDOC CRM should ensure conformance with the relevant data structures. Conformance pertains either to data to be made accessible in an integrated environment or intended for transport to other environments. Any encoding of data in a formal language that preserves the relations of the classes, properties, and inheritance rules defined by this International Standard, is regarded as conformant.

Conformance with the CIDOC CRM does not require complete matching of all local documentation structures, nor that all concepts and structures present in this International Standard be implemented. This International Standard is intended to allow room both for extensions, needed to capture the full richness of cultural documentation, and for simplification, in the interests of economy. A system will be deemed partially conformant if it supports a subset of subclasses and sub properties defined by this International Standard. Designers of the system should publish details of the constructs that are supported.

The focus of the CIDOC CRM is the exchange and mediation of structured information. It does not require the interpretation of unstructured (free text) information into a structured, logical form. Unstructured information is supported, but falls outside the scope of conformance considerations.

Any documentation system will be deemed conformant with this International Standard, regardless of the internal data structures it uses; if a deterministic logical algorithm can be constructed, that transforms data contained in the system into a directly compatible form without loss of meaning.

No assumptions are made as to the nature of this algorithm. "Without loss of meaning" signifies that designers and users of the system are satisfied that the data representation corresponds to the semantic definitions provided by this International Standard.

The following definitions of key terminology used in this document are provided both as an aid to readers unfamiliar with object-oriented modelling terminology, and to specify the precise usage of terms that are sometimes applied inconsistently across the object-oriented modelling community for the purpose of this document. Where applicable, the editors have tried to consistently use terminology that is compatible with that of the Resource Description Framework (RDF),^[3] a recommendation of the World Wide Web Consortium. The editors have tried to find a language, which is comprehensible to the non-computer expert and precise enough for the computer expert so that both understand the intended meaning.

The CIDOC CRM is an ontology in the sense used in computer science. It has been expressed as an object-oriented semantic model, in the hope that this formulation will be comprehensible to both documentation experts and information scientists alike, while at the same time being readily converted to machine-readable formats such as RDF Schema or OWL. It can be implemented in Relational or Object-Oriented schema. CIDOC CRM instances can also be encoded in RDF, JSON LD, XML, OWL among and others

Although the definition of the CIDOC CRM provided here is complete, it is an intentionally compact and concise presentation of the CIDOC CRM’s 81 classes and 160 unique properties. It does not attempt to articulate the inheritance of properties by subclasses throughout the class hierarchy (this would require the declaration of several thousand properties, as opposed to 160). However, this definition does contain all of the information necessary to infer and automatically generate a full declaration of all properties, including inherited properties.

The following naming conventions have been applied throughout the CIDOC CRM:

• Classes are identified by numbers preceded by the letter “E” (historically classes were sometimes referred to as “Entities”), and are named using noun phrases (nominal groups) using title case (initial capitals). For example, E63 Beginning of Existence.
• Properties are identified by numbers preceded by the letter “P,” and are named in both directions using verbal phrases in lower case. Properties with the character of states are named in the present tense, such as “has type”, whereas properties related to events are named in past tense, such as “carried out.” For example, P126 employed (was employed in).
• Property names should be read in their non-parenthetical form for the domain-to-range direction, and in parenthetical form for the range-to-domain direction. Reading a property in range-to-domain direction is equivalent to the inverse of that property. Following a current notational practice in OWL knowledge representation language, we represent inverse properties in this text by adding a letter “i” following the identification number and the parenthetical form of the full property name, such as P59i is located on or within, which is the inverse of P59 has section (is located on or within).
• Properties with a range that is a subclass of E59 Primitive Value (such as E1 CRM Entity. P3 has note: E62 String, for example) have no parenthetical name form, because reading the property name in the range-to-domain direction is not regarded as meaningful.
• Properties that have identical domain and range are either symmetric or transitive. Instantiating a symmetric property implies that the same relation holds for both the domain-to-range and the range-to-domain directions. An example of this is E53 Place. P122 borders with: E53 Place. The names of symmetric properties have no parenthetical form, because reading in the range-to-domain direction is the same as the domain-to-range reading. Transitive asymmetric properties, such as E4 Period. P9 consist of (forms part of): E4 Period, have a parenthetical form that relates to the meaning of the inverse direction.
• The choice of the domain of properties, and hence the order of their names, are established in accordance with the following priority list:
  • Temporal Entity and its subclasses
  • Thing and its subclasses
  • Actor and its subclasses
  • Other

CIDOC CRM is formulated as a class system with inheritance. A property P with domain A and range B will also be a property between any possible subclasses of A and of B. In many cases there will be a common subclass C of both A and B. In these cases, when the property is restricted to C, that is, with C as domain and range, the restricted property could be transitive. For instance, an E73 Information Object can be incorporated into an E90 Symbolic Object and thus an information object can be incorporated in another information object.

In the definition of CIDOC CRM the transitive properties are explicitly marked as such in the scope notes. All unmarked properties should be considered as not transitive.

Some properties are declared as shortcuts of longer, more comprehensively articulated paths that connect the same domain and range classes as the shortcut property via one or more intermediate classes. For example, the property E18 Physical Thing. P52 has current owner (is current owner of): E39 Actor, is a shortcut for a fully articulated path from E18 Physical Thing through E8 Acquisition to E39 Actor. An instance of the fully-articulated path always implies an instance of the shortcut property. However, the inverse may not be true; an instance of the fully-articulated path cannot always be inferred from an instance of the shortcut property inside the frame of the actual KB

The class E13 Attribute Assignment allows for the documentation of how the assignment of any property came about, and whose opinion it was, even in cases of properties not explicitly characterized as “shortcuts”.

The present CIDOC CRM specifications are annotated with logical axioms, providing an additional formal expression of the CIDOC CRM ontology. This section briefly introduces the assumptions that are at the basis of the logical expression of the CIDOC CRM (for a fully detailed account of the logical expression of semantic data modelling, see (Reiter,1984)).

The CIDOC CRM is expressed in terms of the primitives of semantic data modelling. As such, it consists of:

• classes, which represent general notions in the domain of discourse, such as the CIDOC CRM class E21 Person which represents the notion of person;
• properties, which represent the binary relations that link the individuals in the domain of discourse, such as the CIDOC CRM property P152 has parent linking a person to one of the person’s parent.

Classes and properties are used to express ontological knowledge by means of various kinds of constraints, such as sub-class/sub-property links, e.g., E21 Person is a sub-class of E20 Biological Object, or domain/range constraints, e.g., the domain of P152 has parent is class E21 Person.

In contrast, first-order logic-based knowledge representation relies on a language for formally encoding an ontology. This language can be directly put in correspondence with semantic data modelling in a straightforward way:

• classes are named by unary predicate symbols; conventionally, we use E21 as the unary predicate symbol corresponding to class E21 Person;
• properties are named by binary predicate symbols; conventionally, we use P152 as the binary predicate symbol corresponding to property P152 has parent.
• properties of properties, “.1 properties” are named by ternary predicate symbols; conventionally, we use P14.1 as the ternary predicate symbol corresponding to property P14.1 in the role of.

Ontology is expressed in logic by means of logical axioms, which correspond to the constraints of semantic modelling. In the definition of classes and properties of the CIDOC CRM the axioms are placed under the heading ‘In first order logic’. There are several options for writing statements in first order logic. In this document we use a standard compact notation widely used in text books and scientific papers. The definition is given in the table below.

Table 1: Symbolic Operators in First Order Logic Representation

For instance, the above sub-class link between E21 Person and E20 Biological Object can be formulated in first order logic as the axiom:

(∀x) [E21(x) ⇒E20(x)]

(reading: for all individuals x, if x is a E21 then x is an E20).

In the definitions of classes and properties in this document the universal quantifier(s) are omitted for simplicity, so the above axiom is simply written:

E21(x) ⇒E20(x)

Likewise, the above domain constraint on property P152 has parent can be formulated in first order logic as the axiom:

P152(x,y) ⇒E21(x)

(reading: for all individuals x and y, if x is a P152 of y, then x is an E21).

These basic considerations should be used by the reader to understand the logical axioms that are used into the definition of the classes and properties. Further information about the first order formulation of CIDOC CRM can be found in (Meghini & Doerr, 2018)

Quantifiers for properties are provided for the purpose of semantic clarification only, and should not be treated as implementation recommendations. The CIDOC CRM has been designed to accommodate alternative opinions and incomplete information, and therefore all properties should be implemented as optional and repeatable for their domain and range (“many to many (0,n:0,n)”). Therefore, the term “cardinality constraints” is avoided here, as it typically pertains to implementations.

The following table lists all possible property quantifiers occurring in this document by their notation, together with an explanation in plain words. In order to provide optimal clarity, two widely accepted notations are used redundantly in this document, a verbal and a numeric one. The verbal notation uses phrases such as “one to many”, and the numeric one, expressions such as “(0,n:0,1)”. While the terms “one”, “many” and “necessary” are quite intuitive, the term “dependent” denotes a situation where a range instance cannot exist without an instance of the respective property. In other words, the property is “necessary” for its range. (Meghini, C. & Doerr, M., 2018)

The CIDOC CRM defines some dependencies between properties and the classes that are their domains or ranges. These can be one or both of the following:

• the property is necessary for the domain
• the property is necessary for the range, or, in other words, the range is dependent on the property.

The possible kinds of dependencies are defined in the table above. Note that if a dependent property is not specified for an instance of the respective domain or range, it means that the property exists, but the value on one side of the property is unknown. In the case of optional properties, the methodology proposed by the CIDOC CRM does not distinguish between a value being unknown or the property not being applicable at all. For example, one may know that an object has an owner, but the owner is unknown. In a CIDOC CRM instance this case cannot be distinguished from the fact that the object has no owner at all. Of course, such details can always be specified by a textual note.

The following modelling principles have guided and informed the development of the CIDOC CRM.

The CIDOC CRM is a formal ontology in the sense introduced by (Guarino, 1998).^[4] The present document is intended to embrace an audience not specialized in computer science and logic; therefore, it focuses on the informal semantics and on the pragmatics of the CIDOC CRM concepts, offering a detailed discussion of the main traits of the conceptualization underlying the CIDOC CRM through basic usage patterns.^[5] The CIDOC CRM aims to assist sharing, connecting and integrating information from research about the past. In order to understand the function of a formal ontology of this kind, one needs to make the following distinctions:

• The material reality. For the purpose of the CIDOC CRM, material reality is regarded as whatever has substance that can be perceived with senses or instruments. Examples are people, a forest or a settlement environment, sea, atmosphere, distant celestial or cellular micro structures, including what we assume could be potentially or theoretically perceived if we could be there, such as the center of Earth or the sun, and all that is past. It is constrained to space and time. What goes on in our minds or is produced by our minds is also regarded as part of the material reality, as it becomes materially evident to other people at least by our utterances, behavior and products.
• The units of description or particulars, i.e., the things and relations which we refer to in order to distinguish parts of reality. Examples are Mount Ida, the Taj Mahal, the formation of China by emperor Qin Shi Huang (秦始皇) in 221BC, Tut-Ankh Amun and his embalming, Prince Shotoku of Japan sending a mission to China in 607AD, the participation of Socrates in the Battle of Potidaea or the radiocarbon dating of the Iceman Ötzi (Kutschera, 2002).

A formal ontology, such as the CIDOC CRM, constitutes a controlled language for talking about particulars. I.e., it provides classes and properties for categorizing particulars as so-called “instances” in a way that their individuation, unity and relevant properties are as unambiguous as possible. For instance, Tut-Ankh Amun as instance of E21 Person is the real pharaoh from his birth to death, and not extending to his mummy, as follows from the specification of the class E21 Person and its properties in the CIDOC CRM.

For clarification, the CIDOC CRM does not take a position against or in favour of the existence of spiritual substance nor of substance not accessible by either senses or instruments, nor does it suggest a materialistic philosophy. However, for practical reasons, it relies on the priority of integrating information based on material evidence available for whatever human experience. The CIDOC CRM only commits to a unique material reality independent from the observer.

When we provide descriptions of particulars, we need to refer to them by unique names, titles or constructed identifiers, all of which are instances of E41 Appellation in the CIDOC CRM, in order the reference to be independent of the context. (In contrast, reference to particulars by pronouns or enumerations of characteristic properties, such as name and birth date, are context dependent). The appellation, and the relation between the appellation and the referred item or relationship, must not be confused with the referred item and its identity. For example, Tut-Ankh Amun the name (instance of E41 Appellation) is different from Tut-Ankh Amun the person (instance of E21 Person) and also different from the relationship between name and person (P1 is identified by). Instances of CIDOC CRM classes are the real particulars, not their names, but in descriptions, names must be used as surrogates for the real things meant. Particulars are approximate individuations, like sections, of parts of reality. In other words, the uniqueness of reality does not depend on where one draws the line between the mountain and the valley.

A CIDOC CRM-compatible knowledge base (KB) (Meghini & Doerr 2018) is an instance of E73 Information Object in the CIDOC CRM. It contains (data structures that encode) formal statements representing propositions believed to be true in a reality by an observer. These statements use appellations (e.g., http://id.loc.gov/authorities/names/n79066005^[6]) of ontological particulars and of CRM concepts (e.g., P100i died in). Thereby users, in their capacity of having real-world knowledge and cognition, may be able to relate these statements to the propositions they are meant to characterize, and be able to reason and research about their validity. In other words, the formal instances in a knowledge base are the identifiers, not the real things or phenomena. A special case is digital content: a KB in a computer system may contain statements about instances of E90 Symbolic Object and the real thing may be text residing within the same KB. The instance of E90 Symbolic Object and its textual representation are separate entities and they can be connected with the property P190 has symbolic content.

Therefore, a knowledge base does not contain knowledge, but statements that represent knowledge, as long as there exist people that can resolve the identifiers used to their referents. (Appellations described in a knowledge base, and not used as primary substitutes of other items, are of course explicitly declared as instances of E41 Appellation in the knowledge base.)

This section describes a recommended good practice how to relate authority to knowledge base contents.

Statements in a KB must have been inserted by some human agent, either directly or indirectly. However, these statements often make no reference to that agent, lacking attribution of authority. An example of such statements in the CIDOC CRM is information expressed through shortcuts such as P2 has type. In the domain of cultural heritage, it is common practice that the responsibility for maintaining knowledge in the KB is elaborated in institutional policy or protocol documents. Thus, it is reasonable to hold that statements which lack explicit authority attribution can be read as the official view of the administrating institution of that system, i.e., the maintainers of the KB. This does not imply that the knowledge described in the KB is complete. So long as the information is under active management, it remains continuously open to revision and improvement as further research reveals further understandings.^[7] A KB does not represent a slice of reality, but the justified beliefs of its maintainers about that reality. For simplicity, we speak about a KB as representing some reality.

Statements in a KB may also carry explicit references to agents that produced them, i.e., further statements of responsibility. In CIDOC CRM such statements of responsibility are expressed though knowledge creation events such as E13 Attribute Assignment and its relevant subclasses. Any knowledge that is about an explicit knowledge creation event, where the creator’s identity has been given, is implicitly attributed to be correctly referred to in the KB by the maintaining authority, whereas the responsibility for the content created by that event is assigned to the agent identified as causal to that event.

In the special case of an institution taking over stewardship of a database transferred into their custody, two relations of responsibility for the knowledge therein can be envisioned. If the institution accepts the dataset and undertakes to maintain and update it, then they take on responsibility for that information and become the default authority behind its statements as described above. If, on the other hand, the institution accepts the data set and stores it without change as a closed resource, then it can be considered that the default authority remains the original steward like for any other scholarly document kept by the institution.

Since the intended scope of the CIDOC CRM is a subset of the “real” world and is therefore potentially infinite, the model has been designed to be extensible through the linkage of compatible external type hierarchies.

Of necessity, some concepts covered by the CIDOC CRM are defined in less details than others: E39 Actor and E30 Right, for example. This is a natural consequence of staying within the model’s clearly articulated practical scope in an intrinsically unlimited domain of discourse. These ‘underdeveloped’ concepts can be considered as candidate superclasses for compatible extensions, in particular for disciplines with a respective focus. Additions to the model are known as extensions while the main model is known as CRMbase.

Compatibility of extensions with the CRM means that data structured according to an extension must also remain valid as instances of CIDOC CRM base classes. In practical terms, this implies query containment: any queries based on CIDOC CRM concepts to a KB should retrieve a result set that is correct according to the model’s semantics, regardless of whether the KR is structured according to the CIDOC CRM’s semantics alone, or according to the CIDOC CRM plus compatible extensions. For example, a query such as “list all events” should recall 100% of the instances deemed to be events by the CIDOC CRM, regardless of how they are classified by the extension.

A sufficient condition for the compatibility of an extension with the CIDOC CRM is that its classes, other than E1 CRM Entity, subsume all classes of the extension, and all properties of the extension are either subsumed by CRM properties, or are part of a path for which a CIDOC CRM property is a shortcut, and that classes and properties of the extension can be well distinguished from those in the CIDOC CRM. For instance, a class “tangible object” may be in conflict with existing classes of the CIDOC CRM. Obviously, such a condition can only be tested intellectually.

The CRM provides a number of mechanisms to ensure that coverage of the intended scope can be increased on demand without loosing compatibility:

1. Existing classes can be extended, either structurally as subclasses or dynamically using the type hierarchy (see section About Types below).
2. Existing properties can be extended, either structurally as subproperties, or in some cases, dynamically, using properties of properties which allow subtyping (see section About Types below).
3. Additional information that falls outside the semantics formally defined by the CIDOC CRM can be recorded as unstructured data using E1 CRM Entity. P3 has note: E62 String.
4. Extending the CIDOC CRM by superclasses and properties that pertain to a wider scope. They are called conservative extensions, if they preserve backwards compatibility with instances described with the CIDOC CRM.

Following strategies 1, 2 and 3 will have the result that the CIDOC CRM concepts subsume and thereby cover the extensions. This means that querying an extended knowledge base only with concepts of the CIDOC CRM will nevertheless retrieve all facts described via the extensions.

In mechanism 3, the information in the notes is accessible in the respective knowledge base by retrieving the instances of E1 CRM Entity that are domain of P3 has note. Keyword search will also work for the content of the note. Rules should be applied to attach a note to the item most specific for the content. For instance, details about the role of an actor in an activity should be associated with the instance of E7 Activity, and not with the instance of E39 Actor. This approach is preferable when queries relating elements from the content of such notes across the knowledge base are not expected.

In general, only concepts to be used for selecting multiple instances from the knowledge base by formal querying need to be explicitly modelled. This criterion depends on the expected scope and use of the particular knowledge base. The CIDOC CRM prioritizes modelling the kinds of facts one would like to retrieve and relate from heterogeneous content sources, potentially from different institutions. It does not, by way of contrast, focus on the modelling of facts with a more local scope such as the administrative practices internal to an institution.

Mechanism 4, conservative extension, is more complex:

With increasing use of the CIDOC CRM, there is also a need for extensions that model phenomena from a scope wider than the original one of the CIDOC CRM, but which are also applicable to the concepts that do fall within the CIDOC CRM’s scope. When this occurs, properties of the CIDOC CRM may be found to be applicable more generally to superclasses of the extension than to those of their current domain or range in the CIDOC CRM. This is a consequence of the key principle of the CIDOC CRM to model “bottom up”, i.e., selecting the domains and ranges for properties to be as narrow as they would apply in a well understood fashion in the current scope, thus avoiding making poorly understood generalizations at risk of requiring non-monotonic correction.

The fourth mechanism for extending the CIDOC CRM by conservation extension can be seen to be split into two cases:

1. A new class or property is added to an extension of the CIDOC CRM, which is not covered by superclasses other than E1 CRM Entity or a superproperty in the CIDOC CRM respectively. In this case, all facts described only by such concepts are not accessible by queries with CIDOC CRM concepts. Therefore, the extension should publish in a compatibility statement the additional relevant high-level classes and properties needed to retrieve all facts documented with the extended model. This case is a monotonic extension.
2. The domain or range of an existing property in the CIDOC CRM is changed to a superclass of the one or the other or both, because the property is understood to be applicable beyond its originally anticipated scope. In this case, all facts described by the extension are still accessible by querying with the concepts of the CIDOC CRM, but the extension can describe additional facts that the CIDOC CRM could not. This case is a monotonic extension and generally recommended, because it enables bottom-up evolution of the model. If this change is part of a new release of the CIDOC CRM itself, it is simply backwards compatible, and this has been done frequently in the evolution of this model.

If case (2) should be documented and implemented in an extension module separate from the CIDOC CRM, it may come in conflict with the current way knowledge representation languages, such as RDF/OWL, treat it, because in formal logic changing the range or domain of a property is regarded as changing the ontological meaning completely; there is no distinction between the meaning of the property independent of domain and range and the specification of the domain and range. It is, however, similar to what in logic is called a conservative extension of a theory, and necessary for an effective modular management of ontologies.

Therefore, for the interested reader, we describe here a definition of this case in terms of first order logic, which shows how modularity can formally be achieved:

Let us assume a property P defined with domain class A and range class C also holds for a domain class B, superclass of A, and a range class D, superclass of C, in the sense of its ontological meaning in the real world. We describe this situation by introducing an auxiliary formal property P’, defined with domain class B and range class D, and apply the following logic:

A(x) ⇒ B(x)

C(x) ⇒ D(x)

P(x,y) ⇒ A(x)

P(x,y) ⇒ C(y)

P’(x,y) ⇒ B(x)

P’(x,y) ⇒ D(y)

Then, P’ is a conservative extension of P if: A(x) ∧ C(y) ∧ P’(x,y) ⇔ P(x,y)

In other words, a separate extension module may re-declare the respective property with another identifier, preferably using the same label, and implement the above rule.

Although the scope of the CIDOC CRM is very broad, the model itself is constructed as economically as possible:

• CIDOC CRM classes and properties are either primitive, or they are key concepts in the practical scope.
• Complements of CIDOC CRM classes are not declared, because, considering the Open World principle, there are no properties for complements of a class (see Terminology and first consequence of Monotonicity).

A CIDOC CRM class is declared when:

• It is required as the domain or range of a property not appropriate to its superclass.
• It serves as a merging point of two CIDOC CRM class branches via multiple IsA (e.g., E25 Human-Made Feature). When the branch superclasses are used for multiple instantiation of an item, this item is in the intersection of the scopes. The class resulting from multiple IsA should be narrower in scope than the intersection of the scopes of the branch superclasses.
• It is useful as a leaf class (i.e., at the end of a CIDOC CRM branch) to domain communities building CIDOC CRM extensions or matching key domain classes from other models to the CIDOC CRM (e.g., E34 Inscription).

The CIDOC CRM’s primary function is to support the meaningful integration of information in an Open World. The adoption of the Open World principle means that the CIDOC CRM itself must remain fundamentally open and knowledge bases implemented using it should be flexible enough to receive new insights. At the model level, new classes and properties within the CIDOC CRM’s scope may be found in the course of integrating more documentation records or when new kinds of relevant facts come to the attention of its maintainers. At the level of the KBs, the need to add or revise information may arise due to numerous external factors. Research may open new questions; documentation may be directed to new or different phenomena; natural or social evolution may reveal new objects of study.

It is the aim of the maintainers of the CIDOC CRM to respect the Open World principle and to follow the principle of monotonicity. Monotonicity requires that adding new classes and properties to the model or adding new statements to a knowledge base does not invalidate already modelled structures and existing statements.

A first consequence of this commitment, at the level of the model, is that the CIDOC CRM aims to be monotonic in the sense of Domain Theory. That is to say, the existing CIDOC CRM constructs and the deductions made from them should remain valid and well-formed, even as new constructs are added by extensions to the CIDOC CRM. Any extensions should be, under this method, backwards compatible with previous models. The only exception to this rule arises when a previous construct is considered objectively incorrect by the domain experts and thus subjected to corrective revision. Adopting the principle of monotonicity has active consequences for the basic manner in which classes and properties are designed and declared in the CIDOC CRM. In particular, it forbids the declaration of complement classes, i.e., classes solely defined by excluding instances of some other classes.

For example:

FRBRoo (Bekiari et al (eds). 2015) extends the CIDOC CRM. In version 2.4 of FRBRoo, F51 Name Use Activity was declared as a subclass to the CIDOC CRM class E7 Activity. This class was added in order to describe a phenomenon specific to library practice and not considered within CRM base. F51 Name Use Activity describes the practice of an instance of E74 Group adopting and deploying a name within a context for a time-span. The creation of this extension is monotonic because no existing IsA relationship or inheritance of properties in CRM base are compromised and no future extension is ruled out. By way of contrast, if, to handle this situation, a subclass “Other Activity” had been declared, a non-monotonic change would have been introduced. This would be the case because the scope note of a complement class like “Other Activities” would forbid any future declaration of specializations of E7 Activity such as ‘Name Use Activity’. In the case the need arose to declare a particular specialized subclass, a non-monotonic revision would have to be made, since there would be no principled way to decide which instances of ‘Other Activity’ were instances of the new, specialized class and which were not. Such non-monotonic changes are extremely costly to end users, compromising backwards compatibility and long-term integration.

As a second consequence, maintaining monotonicity is also required during revising or augmenting data within a CIDOC CRM compatible system. That is, existing CIDOC CRM instances, their properties and the deductions made from them, should always remain valid and well-formed, even as new instances, regarded as consistent by the domain expert, are added to the system.

For example:

If someone describes correctly that an item is an instance of E19 Physical Object, and later it is correctly characterized as an instance of E20 Biological Object, the system should not stop treating it as an instance of E19 Physical Object. This is achieved by declaring E20 Biological Object as subclass of E19 Physical Object.

This example further demonstrates that the IsA hierarchy of classes and properties can represent characteristic stages of increasing knowledge about some item during the processes of investigation and collection of evidence. Higher level classes can be used to safely classify objects whose precise characteristics are not known in the first instance. An ambiguous biological object may, for example, be classified as only a physical object. Subsequent investigation can reveal its nature as a biological object. A knowledge base constructed with CIDOC CRM classes designed to support monotonic revision allows for seeking physical objects that were not yet recognized as biological ones. This ability to integrate information with different specificity of description in a well-defined way is particularly important for large-scale information integration. Such a system supports scholars being able to integrate all information about potentially relevant phenomena into the information system without forcing an over or under commitment to knowledge about the object. Since large scale information integration always deals with different levels of knowledge of its relevant objects, this feature enables a consistent approach to data integration.

A third consequence, applied at the level of the knowledge base, is that in order to formally preserve monotonicity, when it is required to record and store alternative opinions regarding phenomena all formally defined properties should be implemented as unconstrained (many: many) so that conflicting instances of properties are merely accumulated. Thus, integrated knowledge can serve as a research tool for accumulating relevant alternative opinions around well-defined entities, whereas conclusions about the truth are the task of open-ended scientific or scholarly hypothesis building.

For example:

King Arthur’s basic life events are highly contested. Once entered in a knowledge base, he should be defined as an instance of E21 Person and treated as having existed as such within the sense of our historical discourse. The instance of E21 Person is used as the collection point for describing possible properties and existence of this individual. Alternative opinions about properties, such as the birthplace and his living places, should be accumulated without validity decisions being made during data compilation. King Arthur may be entered as a different instance, of E28 Conceptual Object, for describing him as mythological character and accumulating possibly mythological facts.

The fourth consequence of monotonicity relates to the use of time dependent properties in a knowledge base. Certain properties declared in the CIDOC CRM, such as having a part, an owner or a location, may change many times for a single item during the course of its existence. Asserting that such a property holds for some item means that that property held for some particular, undetermined time-span within the course of its existence. Consequently, one item may be the subject of multiple statements asserting the instantiation of that property without conflict or need for revision. The collection of such statements would reflect an aggregation of these instances of this property holding over the time-span of the item’s existence. If a more specific temporal knowledge is required/available, it is recommended to explicitly describe the events leading to the assertion of that property for that item. For example, in the case of acquiring or losing an item, it would be appropriate to declare the related event class such as E9 Move. By virtue of this principle, the CRM achieves monotonicity with respect to an increase of knowledge about the states of an item at different times, regardless of their temporal order.

Time-neutral properties may be specialized in a future monotonic extension by time-specific properties, but not vice-versa. Also, many properties registered do not change over time or are relative to events in the model already. Therefore, the CIDOC CRM always gives priority to modelling properties as time-neutral, and rather representing changes by events.

However, for some of these properties many databases may describe a “current” state relative to some property, such as “current location” or “current owner”. Using such a “current” state means that the database manager is able to verify the respective reality at the latest date of validity of the database. Obviously, this information is non-monotonic, i.e., it requires deletion when the state changes. In order to preserve a reduced monotonicity, these properties have time-neutral superproperties by which respective instances can be reclassified if the validity becomes unknown or no longer holds. Therefore, the use of such properties in the CRM is only recommended if they can be maintained consistently. Otherwise, they should be reclassified by their time-neutral superproperties. This holds in particular if data is exported to another repository, see also the paragraph “Authorship of Knowledge Base Contents”

Classes are disjoint if they cannot share any common instances at any time, past, present or future. That implies that it is not possible to instantiate an item using a combination of classes that are mutually disjoint or with subclasses of them (see “multiple instantiation” in section “Terminology”). There are many examples of disjoint classes in the CIDOC CRM.

A comprehensive declaration of all possible disjoint class combinations afforded by the CIDOC CRM has not been provided here; it would be of questionable practical utility, and may easily become inconsistent with the goal of providing a concise definition. However, there are two key examples of disjoint class pairs that are fundamental to effective comprehension of the CIDOC CRM:

• E2 Temporal Entity is disjoint from E77 Persistent Item. Instances of the class E2 Temporal Entity are perdurants, whereas instances of the class E77 Persistent Item are endurants. Even though instances of E77 Persistent Item have a limited existence in time, they are fundamentally different in nature from instances of E2 Temporal Entity, because they preserve their identity between events. Declaring endurants and perdurants as disjoint classes is consistent with the distinctions made in data structures that fall within the CIDOC CRM’s practical scope.
• E18 Physical Thing is disjoint from E28 Conceptual Object. The distinction is between material and immaterial items, the latter being exclusively human-made. Instances of E18 Physical Thing and E28 Conceptual Object differ in many fundamental ways; for example, the production of instances of E18 Physical Thing implies the incorporation of physical material, whereas the production of instances of E28 Conceptual Object does not. Similarly, instances of E18 Physical Thing cease to exist when destroyed, whereas an instance of E28 Conceptual Object perishes when it is forgotten or its last physical carrier is destroyed.

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The following paragraphs explain the most general logic of the CIDOC CRM. The CIDOC CRM is a formalized representation of historical discourse, a formal ontology. In this capacity, it is meant to support the (re)presentation of fact based, analytic discourse about what has happened in the past in a human understandable and machine-processable manner. It achieves this function by proposing a series of formalized properties (relations) and classes. The formalized properties support the making of semantically explicit statements relating classes of things. Their formal definition logically explicates the classes of things to which they may pertain. The CIDOC CRM properties thus enable a formal, logically explicit description of relations between individual, real world items, classified under distinct ontological classes. Encoding analytic data pertaining to the past under such a system of statements provides a standard representation for data and allows the uniform application of reasoning to large sets of data.

Grounding this high-level logic is a hierarchical system of classes and relations, that provide basic ontological distinctions by which to represent historical discourse. Familiarity with the basic ontological distinctions made in the top level of the class hierarchy provides the basic entry point to understanding how to apply the CIDOC CRM for knowledge representation.

The highest-level distinction in the CIDOC CRM is represented by the top-level concepts of E77 Persistent Item, equivalent to the philosophical notion of endurant; E2 Temporal Entity, equivalent to the philosophical notion of perdurant and, further, the concept of E92 Spacetime Volume.

As an event-centric model, supporting historical discourse, the CIDOC CRM firstly enables the description of entities that are themselves time-limited processes or evolutions within the passing of time using E2 Temporal Entity and its subclasses. Their basic function is to capture the fact of something having happened over time. In addition to allowing the description of a temporal duration, the subclasses of E2 Temporal Entity are used to document the historical relations between objects, similar to the role of action verbs in a natural language phrase. The more specific subclasses of E2 Temporal Entity enable the documentation of events pertaining to individually related/affected material, social or mental objects that have been described using subclasses of E77 Persistent Item. This precise documentation is enabled through the use of specialized properties formalizing the manner of the relation or affect. Examples of specific subclasses of E2 Temporal Entity include E12 Production, which allows the representation of events of making things by humans, and E5 Event which allows the documentation, among other things, of geological events and large-scale social events such as a war. Each of these subclasses have specific properties associated to them which allow them to function to represent the specific, real world connection between instances of E77 Persistent Item, such as the relation of an object to its time of production through P108 was produced by (E12) or the relation of a place to a geological phenomenon through P7 was place of (E5). The entities that E2 Temporal Entity documents, being time limited processes / occurrences, are such that their existence can be declared only on the basis of direct observation or recording of the event, or indirect observation of its material outcomes. Evidence of such entities may be preserved on material objects that are permanently changed because of them. Likewise, events may have been recorded in text or remembered through oral history. E2 Temporal Entity and its subclasses are central to the CRM and essential for almost all modelling tasks (e.g., in a museum catalogue one cannot consider an object outside its production event).

The real-world entities, which the event centric modelling of the CIDOC CRM aims to enable the accurate historical description of, are captured through E77 Persistent Item and its subclasses. E77 Persistent Item is used to describe entities that are relatively stable in form through the passage of time, maintaining a recognizable identity because their significant properties do not change. Specific subclasses of E77 Persistent Item can illustrate this point. E22 Human Made Object is used for the description of discrete, physical objects having been produced by human action, such as an artwork or monument. An artwork or monument is persistent with regards to its physical constitution. So long as it retains its general physical form, it is said to exist and to participate in the flow of historical events. E28 Conceptual Object is also used to describe persistent items, but of a mental character. It is used to describe identifiable ideas that are named and form an object of historical discourse. Its identity conditions rely in having a carrier by which it can be recalled. The entities described by E77 Persistent Item are prone to change through human activity, biological, geological or environmental processes, but are regarded to continue to exist and be the same just as long as such changes do not alter their basic identity (essence) as defined in the scope note of the relevant class.

The notion of identity is key in the application of CIDOC CRM. The properties and relations it provides are designed to allow the accurate historical description of the evolution of real-world items through time. This being the case, classes and properties are created in order to provide a definition, which will allow the accurate application of the classes or properties to the same real-world items by diverse users. Identity, in the sense of the CIDOC CRM, therefore, means that informed people are able to agree that they refer to the same, single thing in its distinction from others, both in its extent and over its time of existence. The criteria for such a determination should come from understanding the scope note of the respective CIDOC CRM class this thing is regarded to be an instance of, because communication via information systems may not leave space for respective clarifying dialogues between users. For example, the Great Sphinx of Giza may have lost part of its nose, but there is no question that we are still referring to the same monument as that before the damage occurred, since it continues to represent significant characteristics and distinctness from an overall shaping in the past, which is of archaeological relevance. Things lacking sufficient stability or differentiation, such as atmosphere, soil, clouds, waves, are not instances of E77 Persistent Item, and not suited for information integration. Discourse about such items may be documented with concepts of the CIDOC CRM as observations in relation to things of persistent identity, such as places.

Learning to distinguish and then interrelate instances of E77 Persistent Item (endurants) and instances of E2 Temporal Entity (perdurants) using the appropriate properties is key to the proper understanding and application of CIDOC CRM in order to formally represent analytic historical data. In the large majority of cases, the distinction this provides, and the subsequent elaboration of subclasses and properties is adequate to describe the content of database records in the cultural and scientific heritage domain. In exceptional cases, where we need to consider complex combinations of changes of spatial extent over time, the concept of spacetime (E92 Spacetime Volume) also needs to be considered. E92 Spacetime Volume describes the entities whose substance has or is an identifiable, confined geometrical extent in the material world that may vary over time, fuzzy boundaries notwithstanding. For example, the built settlement structure of the city of Athens is confined both from the point of view of time-span (from its founding until now) and from its changing geographical extent over the centuries, which may become more or less evident from current observation, historical documents and excavations. Even though E92 Spacetime Volume is an important theoretical part of the model, it can be ignored for most practical documentation and modelling tasks.

The key to the proper understanding of CIDOC CRM comes through the appropriation of its basic divisions and the logic these represent. It is important to underline that the CIDOC CRM is not intended to function as a classification system or vocabulary tool. The basic class divisions in CIDOC CRM are declared in order to be able to apply distinct properties to these classes and, in so doing, formulate precise, analytic propositions that represent historical realities. The expressive power of CIDOC CRM comes not from the application of classes to classify entities but in the documenting the interrelation of individual historical items through well-defined properties. These properties characteristically cover subjects such as relations of identifying items by names and identifiers; participation of persistent items in temporal entities; location of temporal entities and physical things in space and time; relations of observation and assessment; part-decomposition and structural properties of anything; influence of things and experiences on the activities of people and their products; reference of information objects to anything.

We explain these concepts with the help of graphical representations in the next sections.

Figure 1 illustrates the minimal properties in the CIDOC CRM for documenting “what has happened”, the central pattern of the Model. Let us first consider the class E1 CRM Entity, the formal top class of the model. It primarily serves a technical purpose to aggregate the ontologically meaningful concepts of the model. It declares however two important properties of general validity and distinct features of the Model: P1 is identified by, with range E41 Appellation, makes the fundamental ontological distinction between the identity of a particular and an identifier (see section “Reality, Knowledge Bases and CIDOC CRM” above), and in practice allows for describing a discourse about resolving historical ambiguities of names and reconciliation of multiple identifiers. The property P2 has type, with range E55 Type, constitutes a practical interface for refining classes by terminologies, being often volatile, as detailed in the section “About Types” below.

Figure 1: High Level Properties and Classes of CIDOC CRM

All classes in figure 1 are direct or indirect subclasses of E1 CRM Entity, but for better readability, only the “subclass of” -link from E2 Temporal Entity is shown. The latter comprises phenomena that continuously occur over some time-span (E52 Time-Span) in the natural time dimension, but some of them may not be confined to specific area, such as a marriage status. Further specializing, E4 Period comprises phenomena occurring in addition within a specific area in the physical space, which can be specified by P7 took place at, with range E53 Place. Instances of E4 Period can be of any size, such as the Warring States Period, the Roman Period, a siege or just the process of making a signature. Further specializing, E5 Event comprises phenomena involving and affecting certain instances of E77 Persistent Item in a way characteristic of the kind of process, which can be specified by the property P12 occurred in the presence of. This concept of presence is very powerful: It constrains the existence of the involved things to the respective places within the specified time and implies the potential of passive or active involvement and mutual impact. Via presence, events represent nodes in a network of things meeting in various combinations in the course of time at different places.

The most important specializations of E77 Persistent Item in this context are: E39 Actor, those capable of intentional actions, E18 Physical Thing, having an identity bound to a relative stability of material form, and E28 Conceptual Object, the idealized things that can be recognized but have an identity independent from the materialization on a specific carrier. The property P12 occurred in the presence of has 36 direct and indirect subproperties, relating these and many more subclasses of E5 Event and E77 Persistent Item. Regardless whether a CRM-compatible knowledge base is created with these properties only or with their much more expressive specializations, querying for the above presented five properties will provide answer to all “Who-When-Where-What-How” questions, and allow for retrieving potentially richly elaborated stories of people, places, times and things.

This pattern of “meeting” is complemented by two more subclasses of E5 Event: E63 Beginning of Existence and E64 End of Existence, which imply not only presence, but constitute the endpoints of existence of things and people in space and time, often in explicit presence and interaction with others, be they causal by producing or consuming or just witnessing. Note that the Model supports multiple instantiation. As a consequence, particular events can be instances of combinations of these and others classes, describing tightly integrated processes of multiple nature. The representation of things connected in events by presence, beginning and end of existence is sufficient to describe the logic of termini postquos and antequos, a major form of reasoning about chronology in historical studies.

Example:

As a simple, real example of applying the above concepts we present a historical event, relevant for the history of art: Johann-Joachim Winkelmann (a German Scholar) has seen the so-called Laocoön Group in 1755 in the Vatican in Rome (at display in the Cortile del Belvedere). He described his impressions in 1764 in his “History of the Art of Antiquity”, (being the first to articulate the difference between Greek, Greco-Roman and Roman art, characterizing Greek art with the famous words “…noble simplicity, silent grandeur”). The sculpture, in Hellenistic "Pergamene baroque" style (Bieber 1961, Brilliant 2000) is widely assumed to be a copy, made between 27 BC and 68 AD (following a Roman commission) from a Greek (no more extent) original. Johann-Joachim Winkelmann was born 1717 as child of Martin Winkelmann and Anna-Maria Meyer and died in 1768 in Trieste.

Figure 2 presents a semantic graph of this event, as described above, using CIDOC CRM concepts. The facts in parentheses above are omitted for better clarity. Instances of classes are represented by informative labels instead of identifiers, in boxes showing the class label above the instance label. Properties are represented as arrows with the property label attached. After class labels and property labels, we show in parenthesis the identifiers of the respective superclasses and superproperties from figure 1, in order to demonstrate that the story can be represented and queried with these concepts only. It also shows how concept specialization increases expressiveness without losing genericity. It is noteworthy that the transfer of information from the Greek original, to the copy, to the mind of Winkelmann and into his writings can be solely understood by this chain of things being present in different meetings. Note also that the degree to which a fact is believed to be real does not affect the choice of CIDOC CRM concepts for description of the fact, nor the reality concept underlying the Model.

Figure 2: CIDOC CRM Encoding Example (Winkelmann seeing Laocoön)

Figure 2 represents in addition one more top-level property of the CIDOC CRM: P67 refers to, which describe an evidence-based fact that an information object makes reference to an identifiable item.

As mentioned above, the central concept of the CIDOC CRM is the representation of a part of reality that can be approximated as a network of things meeting in various combinations in spacetime. Using the same example from above, figure 3 illustrates this concept via an alternative symbolic representation. It aims at rendering the idea that people and things in the past have performed mostly unknown trajectories in spacetime and the historical facts known to us constrain their possible whereabouts for some limited time-spans to having been together at some known or unknown place.

We use a one-dimensional representation of space, as used in archaeology to describe the spatial evolution of periods or cultures over time, and a vertical time axis. We symbolize the trajectories of things and people as fuzzy lines between events in order to render their relative indeterminacy between known events. Non-animate things use to be stationary if not transferred, whereas people may move around on their own. We symbolize events as fuzzy ovals to render the fuzzy boundaries of events in space and time. Note that in this representation, as a general pattern, things may “survive” events, emerge from events or end in events. Beginning and ending of existence impose an additional temporal order on events of causal nature, which can be stronger than explicit dates. We symbolize the unreported ends of existence of people and things, which are also events, by a dot at the end of the trajectory.

Figure 3: Symbolic Representation of "Winkelmann seeing Laocoön" as an Evolution in Space and Time

In the following, we give an overview of the system of spatial and temporal relations in the CIDOC CRM, because it constitutes an important tool for precise documentation of the past and has a certain complexity that needs to be understood in a synopsis.

A major area of documentation and historical research centres around positioning in space of what has happened and the things involved, as well as reasoning about respective spatial relations. The key class CIDOC CRM provides for modelling this information is E53 Place. E53 Place is used to document geometric extents in the physical space containing actual or possible positions of things or happenings. The higher-level properties and classes of CIDOC CRM that centre around E53 Place allow for the documentation of: relations between places; recording the geometric expressions defining or approximating a place and their semantic function; tracing the history of locations of a physical object; identifying the places where an individual or group have been located; identifying places on a physical object and the spatial extent of certain temporal entities.

Geometric Expressions of Place: Contemporary documentation of spatial information has access to advanced equipment for accurately recording location and libraries of georeferenced place information. For this reason, documentation of place now often includes the recording of precise coordinates for a referenced place. Of great importance semantically, is to understand the manner in which such a geometric place expression actually relates to a referenced place. The cluster or relations P168 place is defined by, P171 at some place within, and P172 contains allows the user to link to geometric place expressions while also accurately indicating how this expression relates to the documented place. Geometric place expressions are instances of E94 Space Primitive, a primitive class for expressing values in data systems not further analyzed in the CIDOC CRM. These properties provide a valid interface to the OGC standards, as elaborated in CRMgeo (Doerr & Hiebel 2013).

Figure 4: Basic CIDOC CRM Properties and Classes for Reasoning about Spatial Information

Relations between Places: The cluster of relations P89 falls within (contains), P122 borders with, P121 overlaps with and P189 approximates can express relative relationships held between places. These properties hold between instances of E53 Place and allow interordering places using common mereotopological concepts.

History of Object Locations: Instances of place are often referenced in order to record the location of some object. When the movement of the object to different locations through time is of interest, it is also important to be able to analytically record the different locations at which an object was and at what point. The CIDOC CRM offers two top level mechanisms for tracing the relation of objects to places. If the aspect of time is unknown or not of interest, then an object can be related to a place through the properties P53 has former or current location and P55 has current location. The former property is the conservatively appropriate choice for documenting the object-to-place relation when time elements are not known. If one is actively tracking current location, the latter property is also of use. When an accurate history of the temporal aspect of location should be provided, the user should take advantage of the class E9 Move, a temporal entity class. Instantiating E9 Move allows the user to document the origin, destination and concerned object of a move event using the collection of properties P27 moved from, P26 moved to, P25 moved. Being a temporal class E9 Move further allows the tracing of time, agency etc. Note that things may be moved indirectly as parts of or within other things.

Actor Locations: Tracking the history of the location of actors is related to the history of object location with a significant difference: in the CIDOC CRM an actor is defined as an entity featuring agency which is not the case in objects and physical entities in general. Not being physical, an actor cannot be the subject of an instance of E9 Move which documents physical relocations. The CIDOC CRM thus offers the notion of P74 has current or former residence in order to document the relation of a person or group to a location as residing there at some time.

Places on a Physical Object: In the recording of cultural heritage and other scientific data, particularly about mobile objects, including ships, it is often necessary to identify where on an object or a certain feature is located and where a certain phenomenon is observed. For this the CIDOC CRM offers the relation P59 has section relating the object to the places which are defined upon it. Note that Earth is the physical object we relate places to per default. In geological times, a narrower relation to a tectonic plate may be necessary.

Spatial Extent of Temporal Entities: In order to spatially define the extent of temporal phenomena, the CIDOC CRM offers two properties that apply to all instances of temporal entity under the class E4 Period: P7 took place at and P8 took place on or within. The former is used to relate a temporal phenomenon directly to an instance of E53 Place which provides the geometric context in which that phenomenon took place. The latter property allows the documentation of a temporal phenomenon taking place in relation to a physical object. This is useful for recording information such as the occurrence of an event on a moving ship or within a particular storage container, where the geometric location is not known or indirectly relevant.

Historical and scientific discourse about the past deals with different levels of knowledge regarding events and their temporal ordering that feed into chronology. Chronology is fundamental to understanding social and natural history, and reasoning about temporal relations and causality is directly related. An immense wealth of physical observations allows for inferring temporal relations and vice-versa. It is important to be able to document temporality both with regards to known dates but also according to relative positioning within a historical time line. The top-level properties of the CIDOC CRM relating to temporal entities support the documentation of: dates as time spans or dimensions, mereological relations between temporal entities as well as a complete suite of topological relations.

Dates and Durations: When some absolute dates limiting a temporal entity are known, this can be documented by instantiating the P4 has time-span property and creating an instance of E52 Time-span. Dates should then be recorded as instances of E61 Time Primitive and related to the time-span through properties P81 ongoing throughout or P82 at some time within. Time is recorded as a span and not an instant in the CIDOC CRM. The choice of property P81 ongoing throughout allows the documentation of knowledge that a temporal phenomenon was occurring at least at all points of a known time span. The property P82 at some time within allows the weaker claim that the phenomenon must have occurred within the limits of a particular time span without further specifying as to when precisely. It is the default for historical dates, given, for instance, in years for events of much smaller duration. The actual mode of encoding the documented date is outside the scope of the CIDOC CRM, which defines this with a primitive class, E61 Time Primitive. Finally, the property P191 had duration can be deployed in order to document a temporal phenomenon with known duration but with less precisely temporal positioning. For instance, a birth may be known with the precision of a year, but with a duration of 3 hours. For documenting exact time-spans that are result of a declaration rather than observation, for instance in order to describe a time-span multiple events may fall into, the property P170 defines time allows for specifying the time-span uniquely by a temporal primitive, rather than by P81 ongoing throughout or P82 at some time within using an identical time primitive.

Figure 5: Basic CIDOC CRM Properties and Classes for Reasoning about Temporal Information

Mereological relations: The documentation of the part-whole relationship of temporal phenomena is crucial for historical reasoning. The CIDOC CRM distinguishes under temporal entities two immediate specializations: E4 Period is a high-level concept for the documentation of temporal phenomena of change and interactions in space and time, comprising but not limited to historical periods such as Ming or Roman, and is further specialized in rich hierarchy of more specific processes and activities. The second specialization is E3 Condition State, a rather specific class for the documentation of static phases of physical things. The CIDOC CRM so far does not describe a higher-level class of static phases, because they are normally deductions from multiple observations, problematic in information integration and vulnerable to non-monotonic revision. For both classes, two different mereological relations are articulated: The property P9 consists of is used to document proper parthood between instances of E4 Period, i.e., to describe how the phenomena that make up an instance of E4 Period can causally be subdivided into more delimited phenomena. In contrast, the property P10 falls within, explained further in the section about spatiotemporal relations, describes only a non-causal co-occurrence in the same spatiotemporal extent. The property P5 consists of indicates, in analogy, proper parthood between instances of E3 Condition State.

Topological Relations: A lot of semantic relations have implications on the temporal ordering of temporal entities. For instance, meeting someone must occur after birth and before death of the involved parties. Information can only be transferred after it has been learned. On the other side, direct information about temporal order has implications on possible or impossible semantic relations. This form of reasoning is of paramount importance for research about the past. It turned out that the popular temporal relations defined by (Allen, 1983), which the CIDOC CRM had adopted in previous versions, are not well suited to describe inferences from semantic relations, as detailed in the section “Temporal Relation Primitives based on fuzzy boundaries” below. Instead, the CIDOC CRM introduces a theory of fuzzy boundaries in time that enables the accurate interpositioning of temporal entities between themselves taking into account the inherent fuzziness of temporal boundaries. This model subsumes the earlier introduced Allen temporal relations which may continued to be used in extensions of the CIDOC CRM.

Treating space and time as separate entities is normally adequate for describing events and where things are. When more precise documentation and reasoning is required about phenomena spreading out over time, such as Bronze Age, a settlement, a nation, moving reference frames such as ships, things being stored in containers and moved around, built structures being partially destroyed, rebuilt and altered etc., space and time must be understood as a coherent continuum, the so-called spacetime. This is not a familiar concept for many users, and those not interested in such details may therefore skip this section.

However, the respective model the CIDOC CRM adopts constitutes a valid interface to the OGC standards, as elaborated in CRMgeo (Doerr & Hiebel 2013) and important for connecting to GIS applications. The key class CIDOC CRM provides for modelling this information is E92 Spacetime Volume. E92 Spacetime Volume is used to document geometric extents in the physical spacetime containing actual or possible positions of things or happenings, in particular in those cases when the changes of place to be documented cannot be reduced to distinct events, because the spatial extent changes continuously. The higher-level properties and classes of CIDOC CRM that centre around E92 Spacetime Volume allow for the documentation of: relations between spacetime volumes, relations to space and time as separate entities, and treating the exact extent of physical things and periods in space at any time of their existence as spacetime volumes. Its use is particularly elegant for the description of temporal gazetteers.

Defining a Spacetime Volume: There are three ways to define a spacetime volume:

1. the property P169 defines spacetime volume should be used to declare a spatiotemporal container for some things or happenings in terms of spatial coordinates that may vary over time, be it in discrete steps or continuously with the help of spacetime expressions. The latter are instances of E95 Spacetime Primitive, a primitive class for expressing values in data systems not further analyzed in the CIDOC CRM.
2. Instances of E4 Period are regarded to be specialized instances of E92 Spacetime Volume that are formed by the spreading out of the phenomena that make up an instance of E4 Period. As such they are fuzzy but in general observable.
3. The continuous sequence of spatial extent that the matter of an instance of E18 Physical Thing occupies in the course of time, defines a spacetime volume unique to it from the beginning of its existence to its end, which can also be understood as its trajectory through the universe The property P169 defines allows for referring to this spacetime volume, in order to document its additional properties. As such this spacetime volume is fuzzy but in general observable. It is not easy to make a mental picture of the spacetime volume of a physical thing, but the construct simplifies all reasoning about where things have been.

Figure 6: Basic CIDOC CRM Properties and Classes for Reasoning with Spacetime Volumes

Relations with Places and Physical Things: The property P161 has spatial projection associates a spacetime volume with the complete spatial extent it has occupied during its time-span of definition. Due to relativity of space, the definition of an instance of E53 Place must be relative to some physical thing as geometric reference. This can explicitly be documented with the property P157 is at rest relative to. If the place where something is at a certain point in time is given in multiple reference spaces in relative movement, such as with respect to a ship versus to the seafloor, these differently defined places may later move apart. Therefore, a spacetime volume, even though uniquely defined, can have any number of spatial projections, depending on the reference space. Currently, the GPS system defines a default reference space on the surface of Earth. In art conservation and other descriptions of mobile object of fixed shape, it is useful to refer to the precise place a physical thing occupies with respect to itself as reference space via P156 occupies, for further analysis. P156 occupies constitutes a particular projection of the spacetime volume of this thing. In contrast, the property P53 has former or current location only describes that a thing was within a specific place given in some reference space for an undefined time.

Relations with Time-Spans and Periods: The property P160 has temporal projection associates a spacetime volume with the complete temporal extent it has covered comprising all places of its definition. In contrast to places, the reference system of time is unique^[8] except for the choice of origin. For instances of E4 Period and its subclasses, which inherit P160 has temporal projection, the property is actually identical with the property P4 has time span inherited from E2 Temporal Entity, because is describes the temporal extent of the phenomena that make up an instance of E4 Period. Therefore, it is recommended to use P4 has time span for instances of E4 Period and its subclasses, rather than P160 has temporal projection.

Relations of Presence: Instances of E93 Presence are specialized instances of E92 Spacetime Volume that are identical with the spatial evolution of a larger spacetime volume specified by P166 was presence of, but delimited to a, normally short, time-span declared by P164 is temporally specified by. In other words, they constitute “snapshots” or “time-slices” of another spacetime volume, such as the extent of the Roman Empire during 30AD. They are the basic construct to describe exactly where something was or happened at a particular time (-span), in connection with the property P161 has spatial projection. In particular, it allows for describing the whereabouts of mobile objects, be it in the storage of a museum, a palace, deposited in the ground, or transported in a container, such as the bone of a saint. For ease of use, a shortcut P195 was presence of is defined directly to E18 Physical Thing, bypassing the definition of its spacetime volume.

Topological Relations: Finally, the Model defines truly spatiotemporal topological relations. P10 falls within (contains) is the complete inclusion of one spacetime volume in another. It should not be confused with inclusion in the spatial and temporal projection, which may be larger. E.g., in 14 AD, Mesopotamia was not within the Roman Empire. Further, the properties P132 spatiotemporally overlaps with and its negation P133 is spatiotemporally separated from are fundamental to argue about temporary parthood, possible continuity etc.

Virtually all structured descriptions of museum objects begin with a unique object identifier and information about the "type" of the object, often in a set of fields with names like "Classification", "Category", "Object Type", "Object Name", etc. All these fields are used for terms that declare that the object belongs to a particular category of items. In the CIDOC CRM the class E55 Type comprises such terms from thesauri and controlled vocabularies used to characterize and classify instances of CIDOC CRM classes. Instances of E55 Type represent concepts (universals) in contrast to instances of E41 Appellation, which are used to name instances of CIDOC CRM classes.

For this purpose, the CIDOC CRM provides two basic properties that describe classification with terminology, corresponding to what is the current practice in the majority of information systems. The class E1 CRM Entity is the domain of the property P2 has type (is type of), which has the range E55 Type. Consequently, every class in the CIDOC CRM, with the exception of E59 Primitive Value, inherits the property P2 has type (is type of). This provides a general alternative mechanism to specialize the classification of CIDOC CRM instances to any level of detail, by linking to external vocabulary sources, thesauri, classification schemas or ontologies.

Analogous to the function of the P2 has type (is type of) property, some properties in the CIDOC CRM are associated with an additional property. These are numbered in the CIDOC CRM documentation with a ‘.1’ extension. The range of these properties of properties always falls under E55 Type. The purpose of a property of a property is to provide an alternative mechanism to specialize its domain property through the use of property subtypes declared as instances of E55 Type. They do not appear in the property hierarchy list but are included as part of the property declarations and referred to in the class declarations. For example, P62.1 mode of depiction: E55 Type is associated with E24 Physical Man-made Thing. P62 depicts (is depicted by): E1 CRM Entity.

The class E55 Type also serves as the range of properties that relate to categorical knowledge commonly found in cultural documentation. For example, the property P125 used object of type (was type of object used in) enables the CIDOC CRM to express statements such as “this casting was produced using a mould”, meaning that there has been an unknown or unmentioned object, a mould, that was actually used. This enables the specific instance of the casting to be associated with the entire type of manufacturing devices known as moulds. Further, the objects of type “mould” would be related via P2 has type (is type of) to this term. This indirect relationship may actually help in detecting the unknown object in an integrated environment. On the other side, some casting may refer directly to a known mould via P16 used specific object (was used for). So, a statistical question to how many objects in a certain collection are made with moulds could be answered correctly (following both paths through P16 used specific object (was used for) - P2 has type (is type of) and P125 used object of type (was type of object used in). This consistent treatment of categorical knowledge enhances the CIDOC CRM’s ability to integrate cultural knowledge.

Types, that is, instances of E55 Type and its subclasses, can be used to characterize the instances of a CIDOC CRM class and hence refine the meaning of the class. A type ‘artist’ can be used to characterize persons through P2 has type (is type of). On the other hand, in an art history application of the CIDOC CRM it can be adequate to extend the CIDOC CRM class E21 Person with a subclass E21.xx Artist. What is the difference of the type ‘artist’ and the class Artist? From an everyday conceptual point of view there is no difference. Both denote the concept ‘artist’ and identify the same set of persons. Thus, in this setting a type could be seen as a class and the class of types may be seen as a metaclass. Since current systems do not provide an adequate control of user defined metaclasses, the CIDOC CRM prefers to model instances of E55 Type as if they were particulars, with the relationships described in the previous paragraphs.

Users may decide to implement a concept either as a subclass extending the CIDOC CRM class system or as an instance of E55 Type. A new subclass should only be created in case the concept is sufficiently stable and associated with additional explicitly modelled properties specific to it. Otherwise, an instance of E55 Type provides more flexibility of use. Users that may want to describe a discourse not only using a concept extending the CIDOC CRM but also describing the history of this concept itself, may choose to model the same concept both as subclass and as an instance of E55 Type with the same name. Similarly, it should be regarded as good practice to foresee for each term hierarchy refining a CIDOC CRM class a term equivalent of this class as top term. For instance, a term hierarchy for instances of E21 Person may begin with “Person”.

One role of E55 Type is to be the CIDOC CRM’s interface to domain specific ontologies and thesauri or less formal terminological systems. Such sets of concepts can be represented in the CIDOC CRM as subclasses of E55 Type, forming hierarchies of terms, i.e., instances of E55 Type linked via P127 has broader term (has narrower term). Such hierarchies may be extended with additional properties. Other standard models, in particular richer ones, used to describe terminological systems can also be interfaced with the CIDOC CRM by declaring their respective concept class as being equivalent to E55 Type, and their respective broader/narrower relation as being identical with P127 has broader term (has narrower term), as long as they are semantically compatible.

In addition to being an interface to external thesauri and classification systems, E55 Type is an ordinary class in the CIDOC CRM and a subclass of E28 Conceptual Object. E55 Type and its subclasses inherit all properties from this superclass. Thus, together with the CIDOC CRM class E83 Type Creation the rigorous scholarly or scientific process that ensures a type is exhaustively described and appropriately named can be modelled inside the CIDOC CRM. In some cases, particularly in archaeology and the life sciences, E83 Type Creation requires the identification of an exemplary specimen and the publication of the type definition in an appropriate scholarly forum. This is very central to research in the life sciences, where a type would be referred to as a “taxon,” the type description as a “protologue,” and the exemplary specimens as “original element” or “holotype”.

Finally, instances of E55 Type or suitable subclasses can describe universals from type systems not organized in thesauri or ontologies, such as industrial product names and types, defined and published by the producers themselves for each new product or product variant.

It is characteristic for sciences dealing with the past, such as history, archaeology or geology, to derive temporal topological relations from stratigraphic and other observations and from considerations of causality between events. For this reason, the CIDOC CRM introduced in version 3.3 the whole set of temporal relationships of Allen’s temporal logic (the now deprecated properties P114 to P120). It was regarded at that time as a well-justified, exhaustive and sufficient theory to deal with temporal topological relationships of spatiotemporal phenomena relevant to cultural historical discourse. Allen’s temporal logic is based on the assumption of known, exact endpoints of time intervals (time-spans), described by an exhaustive set of mutually exclusive relationships.

Since many temporal relations can be inferred from facts causal to them, e.g., a birth necessarily occurring before any intentional interaction of a person with other individuals, or from observations of material evidence without knowing the absolute time, the temporal relationships pertain in the CIDOC CRM to E2 Temporal Entities, and not their Time-Spans, which require knowledge of absolute time. If absolute times are known, deduction of Allen’s relation is a simple question of automated calculus and not the kind of primary scientific insight the CIDOC CRM, as a core model, is interested in. However, their application turned out to be problematic in practice for two reasons:

Firstly, facts causal to temporal relationships result in expressions that often require a disjunction (logical OR condition) of Allen’s relationships. For instance, a child may be stillborn. Ignoring states at pregnancy as it is usual in older historical sources, birth may be equal to death, meet with death or be before death. The knowledge representation formalism chosen for the CIDOC CRM however does not allow for specifying disjunctions, except within queries. Consequently, simple properties of the CIDOC CRM that imply a temporal order, such as P134 continued, cannot be declared as subproperties of the temporal relationship they do imply, which would be, in this case: “before, meets, overlaps, starts, started-by, contains, finishes, finished-by, equals, during or overlapped by” (see P174 starts before the end of).

Secondly, nature does not allow us to observe equality of points in time. There are three possible interpretations to this fact. Common to all three interpretations is that they can be described in terms of fuzzy boundaries. The model proposed here is consistent with all three of these interpretations.

1. Any observable phenomenon that can be dated has a natural temporal extent with fuzzy boundaries of gradual transition from not existing to definitely existing and then to no longer existing.
2. These fuzzy boundaries can also be interpreted as the time intervals about which experts, even with a complete knowledge of the described phenomenon, may not agree as to whether this phenomenon is already ongoing or not, or still ongoing or not.
3. Under a third interpretation, the fact that an instance of E2 Temporal Entity is ongoing is not observable within the fuzzy boundaries.

Consider, for instance, a birth. Extending over a limited and non-negligible duration in the scale of hours it begins and ends gradually (1), but can be given alternative scientific definitions of start and end points (2), and neither of these can be determined with a precision much smaller than on a scale of minutes (3). The fuzzy boundaries do not describe the relation of incomplete or imprecise knowledge to reality. Assuming a lowest granularity in time is an approach which does not help, because the relevant extent of fuzziness varies at a huge scale even in cultural reasoning, depending on the type of phenomena considered. The only exact match is between arbitrarily declared time intervals, such as the end of a year being equal to the beginning of the next year, or that “Early Minoan” ends exactly when “Middle Minoan” starts, whenever that might have been. Consequently, we introduce here a new set of “temporal relation primitives” with the following characteristics:

• It is a minimal set of properties that allows for specifying all possible relations between two time intervals given by their start and end points, either directly, or by conjunction (logical AND condition) of the latter.
• Start and end points are interpreted as “thick” fuzzy boundaries as described above.
• Conditions of equality of end points are relaxed to the condition that the fuzzy boundaries overlap. Therefore, knowledge of the shape of the fuzzy function is not needed.
• All of Allen’s relationships can be expressed either directly or by conjunctions of these properties.
• In case of time intervals without or with negligibly short fuzzy boundaries, all of Allen’s relationships can exactly be described by adequate conjunctions of these properties.
• No relationship is equal to the inverse of another. Inverses are specified by exchanging the roles of domain and range.

Notation

We use the following notation:

Comparing two instances of E2 Temporal Entity, we denote one with capital letter A, its (fuzzy) starting time with A^start and its (fuzzy) ending time with A^end, such that A = [A^start,A^end]; we denote the other with capital letter B, its (fuzzy) starting time with B^start and its (fuzzy) ending time with B^end, such that B = [B^start,B^end].

We identify a temporal relation with a predicate name (label) and define it by one or more (in)equality expressions between its end points, such as:

A starts before the end of B if and only if (⇔) A^start < B^end

We visualize a temporal relation symbolizing the temporal extents of two instances A and B of E2 Temporal Entity as horizontal bars, considered to be on a horizontal time-line proceeding from left to right. The fuzzy boundary areas are symbolized by an increasing/decreasing colour gradient. The different choices of relative arrangement the relationship allows for are symbolized by two extreme allowed positions of instance A with respect to instance B connected by arrows. The reader may imagine it as the relative positions of a train A approaching a station B. If the relative length of A compared to B matters, two diagrams are provided.

Figure 7: Explanation of Interior and Boundary and an Example of Use from P174 starts before the end of (ends after the start of).

Overview of Temporal Relation Primitives

The final set of temporal relation primitives can be separated into two groups:

1. Those based on improper inequalities, such as A^start ≤ B^end (odd number items in the list below- table 1).
2. Those based on proper inequalities, such as A^start < B^end (even number items in the list below- table 1).

Improper inequalities with fuzzy boundaries are understood as extending into situations in which the fuzzy boundaries of the respective endpoints may overlap. In other words, they include situations in which it cannot be decided when one interval has ended and when the other started, but there is no knowledge of a definite gap between these endpoints. In a proper inequality with fuzzy boundaries, the fuzzy boundaries of the respective endpoints must not overlap, i.e., there is knowledge of a definite gap between these endpoints, for instance, a discontinuity between settlement phases based on the observation of archaeological layers.

Table 2: Temporal Relation Primitives

Although they do not provide comprehensive definitions, compact mono-hierarchical presentations of the class and property IsA hierarchies have been found to significantly aid comprehension and navigation of the CIDOC CRM. Since the CRM is poly-hierarchical, a mono-hierarchical presentation form is achieved by a top-down expansion of all inverse IsA relations regardless whether a concept has already be presented at another place in the same hierarchy. This form is provided below.

The class hierarchy presented below has the following format:

• Each line begins with a unique class identifier, consisting of a number preceded by the letter “E” (originally denoting “entity,” although now replaced by convention with the term “class”).
• A series of hyphens (“-”) follows the unique class identifier, indicating the hierarchical position of the class in the IsA hierarchy.
• The English name of the class appears to the right of the hyphens.
• The index is ordered by hierarchical level, in a “depth first” manner, from the smaller to the larger subhierarchies.
• Classes that appear in more than one position in the class hierarchy as a result of multiple inheritance are shown in an italic typeface.

The property hierarchy presented below has the following format:

• Each line begins with a unique property identifier, consisting of a number preceded by the letter “P” (for “property”).
• A series of hyphens (“-”) follows the unique property identifier, indicating the hierarchical position of the property in the IsA hierarchy.
• The English name of the property appears to the right of the hyphens, followed by its inverse name in parentheses for reading in the range to domain direction.
• The domain class for which the property is declared.
• The range class that the property references.
• The index is ordered by hierarchical level, in a “depth first” manner, from the smaller to the larger subhierarchies, and by property number between equal siblings.
• Properties that appear in more than one position in the property hierarchy as a result of multiple inheritance are shown in an italic typeface.

The classes of the CIDOC CRM are comprehensively declared in this section using the following format:

• Class names are presented as headings in bold face, preceded by the class’ unique identifier;
• The line “Subclass of:” declares the superclass of the class from which it inherits properties;
• The line “Superclass of:” is a cross-reference to the subclasses of this class;
• The line “Scope note:” contains the textual definition of the concept the class represents;
• The line “Examples:” contains a bulleted list of examples of instances of this class. If the example is also instance of a subclass of this class, the unique identifier of the subclass is added in parenthesis. If the example instantiates two classes, the unique identifiers of both classes is added in parenthesis. Non-fictitious examples may be followed by an explanation in brackets.
• The line “Properties:” declares the list of the class’ properties;
• Each property is represented by its unique identifier, its forward and reverse names, and the range class that it links to, separated by colons;
• Inherited properties are not represented;
• Properties of properties are provided indented and in parentheses beneath their respective domain property.

This class comprises all things in the universe of discourse of the CIDOC Conceptual Reference Model.

It is an abstract concept providing for three general properties:

• Identification by name or appellation, and in particular by a preferred identifier
• Classification by type, allowing further refinement of the specific subclass an instance belongs to
• Attachment of free text and other unstructured data for the expression of anything not captured by formal properties

All other classes within the CIDOC CRM are directly or indirectly specialisations of E1 CRM Entity.

• the earthquake in Lisbon 1755 (E5) (Chester, 2001)

• E1(x)

This class comprises all phenomena, such as the instances of E4 Periods and E5 Events, which happen over a limited extent in time. This extent in time must be contiguous, i.e., without gaps. In case the defining kinds of phenomena for an instance of E2 Temporal Entity cease to happen, and occur later again at another time, we regard that the former instance of E2 Temporal Entity has ended and a new instance has come into existence. In more intuitive terms, the same event cannot happen twice.

In some contexts, such phenomena are also called perdurants. This class is disjoint from E77 Persistent Item and is an abstract class that typically has no direct instances. E2 Temporal Entity is specialized into E4 Period, which applies to a particular geographic area (defined with a greater or lesser degree of precision), and E3 Condition State, which applies to instances of E18 Physical Thing.

• Bronze Age (E4) (Childe, 1963)
• the earthquake in Lisbon 1755 (E5) (Chester, 2001)
• the Peterhof Palace near Saint Petersburg being in ruins from 1944 – 1946 (E3) (Maddox, 2015)

• E2(x) ⇒ E1(x)

This class comprises the states of objects characterised by a certain condition over a time-span.

An instance of this class describes the prevailing physical condition of any material object or feature during a specific instance of E52 Time Span. In general, the time-span for which a certain condition can be asserted may be shorter than the real time-span, for which this condition held.

The nature of that condition can be described using P2 has type. For example, the instance of E3 Condition State “condition of the SS Great Britain between 22 September 1846 and 27 August 1847” can be characterized as an instance “wrecked” of E55 Type.

• the "reconstructed" state of the “Amber Room” in Tsarskoje Selo from summer 2003 until now (Owen, 2009)
• the "ruined" state of Peterhof Palace near Saint Petersburg from 1944 to 1946 (Maddox, 2015)
• the state of my turkey in the oven at 14:30 on 25 December, 2002 [P2 has type: E55 Type “still not cooked”] (fictitious)
• the topography of the leaves of Sinai Printed Book 3234.2361 on the 10th of July 2007 [described as: of type "cockled"] (fictitious)

• E3(x) ⇒ E2(x)

This class comprises sets of coherent phenomena or cultural manifestations occurring in time and space.

It is the social or physical coherence of these phenomena that identify an instance of E4 Period and not the associated spatiotemporal extent. This extent is only the “ground” or space in an abstract physical sense that the actual process of growth, spread and retreat has covered. Consequently, different periods can overlap and coexist in time and space, such as when a nomadic culture exists in the same area and time as a sedentary culture. This also means that overlapping land use rights, common among first nations, amounts to overlapping periods.

Often, this class is used to describe prehistoric or historic periods such as the “Neolithic Period”, the “Ming Dynasty” or the “McCarthy Era”, but also geopolitical units and activities of settlements are regarded as special cases of E4 Period. However, there are no assumptions about the scale of the associated phenomena. In particular all events are seen as synthetic processes consisting of coherent phenomena. Therefore, E4 Period is a superclass of E5 Event. For example, a modern clinical birth, an instance of E67 Birth, can be seen as both a single event, i.e., an instance of E5 Event, and as an extended period, i.e., an instance of E4 Period, that consists of multiple physical processes and complementary activities performed by multiple instances of E39 Actor.

As the actual extent of an instance of E4 Period in spacetime we regard the trajectories of the participating physical things during their participation in an instance of E4 Period. This includes the open spaces via which these things have interacted and the spaces by which they had the potential to interact during that period or event in the way defined by the type of the respective period or event. Examples include the air in a meeting room transferring the voices of the participants. Since these phenomena are fuzzy, we assume the spatiotemporal extent to be contiguous, except for cases of phenomena spreading out over islands or other separated areas, including geopolitical units distributed over disconnected areas such as islands or colonies.

Whether the trajectories necessary for participants to travel between these areas are regarded as part of the spatiotemporal extent or not has to be decided in each case based on a concrete analysis, taking use of the sea for other purposes than travel, such as fishing, into consideration. One may also argue that the activities to govern disconnected areas imply travelling through spaces connecting them and that these areas hence are spatially connected in a way, but it appears counterintuitive to consider for instance travel routes in international waters as extensions of geopolitical units.

Consequently, an instance of E4 Period may occupy a number of disjoint spacetime volumes, however there must not be a discontinuity in the timespan covered by these spacetime volumes. This means that an instance of E4 Period must be contiguous in time. If it has ended in all areas, it has ended as a whole. However, it may end in one area before another, such as in the Polynesian migration, and it continues as long as it is ongoing in at least one area.

We model E4 Period as a subclass of E2 Temporal Entity and of E92 Spacetime Volume. The latter is intended as a phenomenal spacetime volume as defined in CIDOC CRMgeo (Doerr & Hiebel, 2013). By virtue of this multiple inheritance, we can discuss the physical extent of an instance of E4 Period without representing each instance of it together with an instance of its associated spacetime volume. This model combines two quite different kinds of substance: an instance of E4 Period is a phenomenon while an instance of E92 Spacetime Volume is an aggregation of points in spacetime. However, the real spatiotemporal extent of an instance of E4 Period is regarded to be unique to it due to all its details and fuzziness; its identity and existence depends uniquely on the identity of the instance of E4 Period. Therefore, this multiple inheritance is unambiguous and effective and furthermore corresponds to the intuitions of natural language.

Typical use of this class in cultural heritage documentation is for documenting cultural and artistic periods. There are two different conceptualisations of ‘artistic style’, defined either by physical features or by historical context. For example, “Impressionism” can be viewed as a period in the European sphere of influence lasting from approximately 1870 to 1905 during which paintings with particular characteristics were produced by a group of artists that included (among others) Monet, Renoir, Pissarro, Sisley and Degas. Alternatively, it can be regarded as a style applicable to all paintings sharing the characteristics of the works produced by the Impressionist painters, regardless of historical context. The first interpretation is an instance of E4 Period, and the second defines morphological object types that fall under E55 Type.

A geopolitical unit as a specific case of an instance of E4 Period is the set of activities and phenomena related to the claim of power, the consequences of belonging to a jurisdictional area and an administrative system that establishes a geopolitical unit. Examples from the modern period are countries or administrative areas of countries such as districts whose actions and structures define activities and phenomena in the area that they intend to govern. The borders of geopolitical units are often defined in contracts or treaties although they may deviate from the actual practice. The spatiotemporal properties of Geopolitical units can be modelled through the properties inherited from E92 Spacetime Volume.

Another specific case of an instance of E4 Period is the actual extent of the set of activities and phenomena as evidenced by their physical traces that define a settlement, such as the populated period of Nineveh.

• Jurassic (Hallam, 1975)
• Populated Period of Nineveh
• Imperial Rome under Marcus Aurelius
• European Bronze Age (Harrison, 2004)
• Italian Renaissance (Macdonald, 1992)
• Thirty Years War (Lee, 1991)
• Sturm und Drang (Berkoff, 2013)
• Cubism (Cox, 2000)

• E4(x) ⇒ E2(x)
• E4(x) ⇒ E92(x)

This class comprises distinct, delimited and coherent processes and interactions of a material nature, in cultural, social or physical systems, involving and affecting instances of E77 Persistent Item in a way characteristic of the kind of process. Typical examples are meetings, births, deaths, actions of decision taking, making or inventing things, but also more complex and extended ones such as conferences, elections, building of a castle, or battles.

While the continuous growth of a tree lacks the limits characteristic of an event, its germination from a seed does qualify as an event. Similarly, the blowing of the wind lacks the distinctness and limits of an event, but a hurricane, flood or earthquake would qualify as an event. Mental processes are considered as events, in cases where they are connected with the material externalization of their results; for example, the creation of a poem, a performance or a change of intention that becomes obvious from subsequent actions or declarations.

The effects of an instance of E5 Event may not lead to relevant permanent changes of properties or relations of the items involved in it, for example an unrecorded performance. Of course, in order to be documented, some kind of evidence for an event must exist, be it witnesses, traces or products of the event.

While instances of E4 Period always require some form of coherence between its constituent phenomena, in addition, the essential constituents of instances of E5 Event should contribute to an overall effect; for example, the statements made during a meeting and the listening of the audience.

Viewed at a coarse level of detail, an instance of E5 Event may appear as if it had an ‘instantaneous’ overall effect, but any process or interaction of material nature in reality have an extent in time and space. At a fine level, instances of E5 Event may be analyzed into component phenomena and phases within a space and timeframe, and as such can be seen as a period, regardless of the size of the phenomena. The reverse is not necessarily the case: not all instances of E4 Period give rise to a noteworthy overall effect and are thus not instances of E5 Event.

• the birth of Cleopatra (E67) (Pomeroy, 1984)
• the destruction of Herculaneum by volcanic eruption in 79 AD (E6) (Camardo, 2013)
• World War II (E7) (Barber, 1994)
• the Battle of Stalingrad (E7) (Hoyt, 1993)
• the Yalta Conference (E7) (Harbutt, 2010)
• my birthday celebration 28-6-1995 (E7)
• the falling of a tile from my roof last Sunday (fictitious)
• the CIDOC conference 2003 (E7)

• E5(x) ⇒ E4(x)

This class comprises events that destroy one or more instances of E18 Physical Thing such that they lose their identity as the subjects of documentation.

Some destruction events are intentional, while others are independent of human activity. Intentional destruction may be documented by classifying the event as both an instance of E6 Destruction and of E7 Activity.

The decision to document an object as destroyed, transformed or modified is context sensitive:

1. If the matter remaining from the destruction is not documented, the event is modelled solely as an instance of E6 Destruction.
2. An event should also be documented as an instance of E81 Transformation if it results in the destruction of one or more objects and the simultaneous production of others using parts or material from the original. In this case, the new items have separate identities. Matter is preserved, but identity is not.
3. When the initial identity of the changed instance of E18 Physical Thing is preserved, the event should be documented as an instance of E11 Modification.

• the destruction of Herculaneum by volcanic eruption in 79 AD (Camardo, 2013)
• the destruction of Nineveh (E6, E7) (River, 2016)
• the breaking of a champagne glass yesterday by my dog (fictitious)

• E6(x) ⇒ E64(x)

This class comprises actions intentionally carried out by instances of E39 Actor that result in changes of state in the cultural, social, or physical systems documented.

This notion includes complex, composite and long-lasting actions such as the building of a settlement or a war, as well as simple, short-lived actions such as the opening of a door.

• the Battle of Stalingrad (Hoyt, 1993)
• the Yalta Conference (Harbutt, 2010)
• my birthday celebration 28-6-1995
• the writing of “Faust” by Goethe (E65) (Williams, 2020)
• the formation of the Bauhaus 1919 (E66) (Droste, 2006)
• calling the place identified by TGN ‘7017998’ ‘Quyunjig’ by the people of Iraq
• Kira Weber working in glass art from 1984 to 1993
• Kira Weber working in oil and pastel painting from 1993

• E7(x) ⇒ E5(x)

This class comprises transfers of legal ownership from one or more instances of E39 Actor to one or more other instances of E39 Actor.

The class also applies to the establishment or loss of ownership of instances of E18 Physical Thing. It does not, however, imply changes of any other kinds of right. The recording of the donor and/or recipient is optional. It is possible that in an instance of E8 Acquisition there is either no donor or no recipient. Depending on the circumstances, it may describe:

1. the beginning of ownership
2. the end of ownership
3. the transfer of ownership
4. the acquisition from an unknown source
5. the loss of title due to destruction of the item

It may also describe events where a collector appropriates legal title, for example by annexation or field collection. The interpretation of the museum notion of "accession" differs between institutions. The CIDOC CRM therefore models legal ownership (E8 Acquisition) and physical custody (E10 Transfer of Custody) separately. Institutions will then model their specific notions of accession and deaccession as combinations of these.

• the collection of a hammer-head shark of the genus Sphyrna Rafinesque, 1810 (Carchariniformes) by John Steinbeck and Edward Ricketts at Puerto Escondido in the Gulf of Mexico on March 25th, 1940. (Steinbeck, 2000)
• the acquisition of El Greco’s painting entitled ‘The Apostles Peter and Paul’ by the State Hermitage in Saint Petersburg. (https://hermitagemuseum.org/wps/portal/hermitage/digital-collection/01.+Paintings/32730)
• the loss of my stuffed chaffinch ‘Fringilla coelebs Linnaeus, 1758’ due to insect damage last year (fictitious)

• E8(x) ⇒ E7(x)

This class comprises changes of the physical location of the instances of E19 Physical Object.

Note, that the class E9 Move inherits the property P7 took place at (witnessed): E53 Place. This property should be used to describe the trajectory or a larger area within which a move takes place, whereas the properties P26 moved to (was destination of), P27 moved from (was origin of) describe the start and end points only. Moves may also be documented to consist of other moves (via P9 consists of (forms part of)), in order to describe intermediate stages on a trajectory. In that case, start and end points of the partial moves should match appropriately between each other and with the overall event.

• the relocation of London Bridge from the UK to the USA. (Wildfang, 2005)
• the movement of the exhibition “Tutankhamun: Treasures of the Golden Pharaoh” between 15^th September and 2^nd November 2019.

• E9(x) ⇒ E7(x)

This class comprises transfers of the physical custody or the legal responsibility for the physical custody of objects. The recording of the donor or recipient is optional. It is possible that in an instance of E10 Transfer of Custody there is either no donor or no recipient.

Depending on the circumstances, it may describe:

1. the beginning of custody (there is no previous custodian)
2. the end of custody (there is no subsequent custodian)
3. the transfer of custody (transfer from one custodian to the next)
4. the receipt of custody from an unknown source (the previous custodian is unknown)
5. the declared loss of an object (the current or subsequent custodian is unknown)

In the event that only a single kind of transfer of custody occurs, either the legal responsibility for the custody or the actual physical possession of the object but not both, this difference should be expressed using the property P2 has type (is type of).

The sense of physical possession requires that the object of custody be in the hands of the keeper at least with a part representative for the whole. The way, in which a representative part is defined, should ensure that it is unambiguous who keeps a part and who the whole and should be consistent with the identity criteria of the kept instance of E18 Physical Thing.

The interpretation of the museum notion of "accession" differs between institutions. The CIDOC CRM therefore models legal ownership and physical custody separately. Institutions will then model their specific notions of accession and deaccession as combinations of these.

Theft is a specific case of illegal transfer of custody.

• the delivery of the paintings by Secure Deliveries Inc. to the National Gallery
• the return of Picasso’s “Guernica” to Madrid’s Prado in 1981 (Chipp, 1988)

• E10(x) ⇒ E7(x)

This class comprises instances of E7 Activity that are undertaken to create, alter or change instances of E24 Physical Human-Made Thing.

This class includes the production of an item from raw materials and other so far undocumented objects. It also includes the conservation treatment of an object.

Since the distinction between modification and production is not always clear, modification is regarded as the more generally applicable concept. This implies that some items may be consumed or destroyed in an instance of E11 Modification, and that others may be produced as a result of it. An event should also be documented using an instance of E81 Transformation if it results in the destruction of one or more objects and the simultaneous production of others using parts or material from the originals. In this case, the new items have separate identities.

An activity undertaken on an object which was designed to alter it, but which, in fact, it did not in any seemingly significant way (such as the application of a solvent during conservation which failed to dissolve any part of the object), is still considered as an instance of E11 Modification. Typically, any such activity will leave at least forensic traces of evidence on the object.

If the instance of E29 Design or Procedure utilized for the modification prescribes the use of specific materials, they should be documented using property P68 foresees use of (use foreseen by): E57 Material of E29 Design or Procedure, rather than via P126 employed (was employed in): E57 Material.

• the construction of the SS Great Britain (E12) (Gregor, 1971)
• the impregnation of the Vasa warship in Stockholm for preservation after 1956 (Håfors, 2010)
• the transformation of the Enola Gay into a museum exhibit by the National Air and Space Museum in Washington DC between 1993 and 1995 (E12, E81) (Yakel, 2000)
• the last renewal of the gold coating of the Toshogu shrine in Nikko, Japan (Cali and Dougil, 2012)

• E11(x) ⇒ E7(x)

This class comprises activities that are designed to, and succeed in, creating one or more new items.

It specializes the notion of modification into production. The decision as to whether or not an object is regarded as new is context sensitive. Normally, items are considered “new” if there is no obvious overall similarity between them and the consumed items and material used in their production. In other cases, an item is considered “new” because it becomes relevant to documentation by a modification. For example, the scribbling of a name on a potsherd may make it a voting token. The original potsherd may not be worth documenting, in contrast to the inscribed one.

This entity can be collective: the printing of a thousand books, for example, would normally be considered a single event.

An event should also be documented using an instance of E81 Transformation if it results in the destruction of one or more objects and the simultaneous production of others using parts or material from the originals. In this case, the new items have separate identities and matter is preserved, but identity is not.

• the construction of the SS Great Britain (Gregor, 1971)
• the first casting of the Little Mermaid from the harbour of Copenhagen (Dewey, 2003)
• Rembrandt’s creating of the seventh state of his etching “Woman sitting half dressed beside a stove”, 1658, identified by Bartsch Number 197 (E12, E65, E81) (Hind, 1923)

• E12(x) ⇒ E11(x)
• E12(x) ⇒ E63(x)

This class comprises the actions of making assertions about one property of an object or any single relation between two items or concepts. The type of the property asserted to hold between two items or concepts can be described by the property P177 assigned property type: E55 Type.

For example, the class describes the actions of people making propositions and statements during certain scientific/scholarly procedures, e.g., the person and date when a condition statement was made, an identifier was assigned, the museum object was measured, etc. Which kinds of such assignments and statements need to be documented explicitly in structures of a schema rather than free text, depends on whether this information should be accessible by structured queries.

This class allows for the documentation of how the respective assignment came about, and whose opinion it was. Note that all instances of properties described in a knowledge base are the opinion of someone. Per default, they are the opinion of the team maintaining the knowledge base. This fact must not individually be registered for all instances of properties provided by the maintaining team, because it would result in an endless recursion of whose opinion was the description of an opinion. Therefore, the use of instances of E13 Attribute Assignment marks the fact, that the maintaining team is in general neutral to the validity of the respective assertion, but registers someone else’s opinion and how it came about.

All properties assigned in such an action can also be seen as directly relating the respective pair of items or concepts. Multiple use of instances of E13 Attribute Assignment may possibly lead to a collection of contradictory values.

All cases of properties in this model that are also described indirectly through a subclass of E13 Attribute Assignment are characterised as "short cuts" of a path via this subclass. This redundant modelling of two alternative views is preferred because many implementations may have good reasons to model either the action of assertion or the short cut, and the relation between both alternatives can be captured by simple rules.

• the examination of MS Sinai Greek 418 by Nicholas Pickwoad in November 2003 (Honey & Pickwoad , 2010)
• the assessment of the current ownership of Martin Doerr’s silver cup in February 1997 (fictitious)

• E13(x) ⇒ E7(x)

This class describes the act of assessing the state of preservation of an object during a particular period.

The condition assessment may be carried out by inspection, measurement or through historical research. This class is used to document circumstances of the respective assessment that may be relevant to interpret its quality at a later stage, or to continue research on related documents.

• last year’s inspection of humidity damage to the frescos in the St. George chapel in our village (fictitious)
• the condition assessment of the endband cores of MS Sinai Greek 418 by Nicholas Pickwoad in November 2003 (Honey & Pickwoad, 2010)
• the condition assessment of the cover of MS Sinai Greek 418 by Nicholas Pickwoad in November 2003 (Honey & Pickwoad, 2010)

• E14(x) ⇒ E13(x)

This class comprises activities that result in the allocation of an identifier to an instance of E1 CRM Entity. Instances of E15 Identifier Assignment may include the creation of the identifier from multiple constituents, which themselves may be instances of E41 Appellation. The syntax and kinds of constituents to be used may be declared in a rule constituting an instance of E29 Design or Procedure.

Examples of such identifiers include Find Numbers, Inventory Numbers, uniform titles in the sense of librarianship and Digital Object Identifiers (DOI). Documenting the act of identifier assignment and deassignment is especially useful when objects change custody or the identification system of an organization is changed. In order to keep track of the identity of things in such cases, it is important to document by whom, when and for what purpose an identifier is assigned to an item.

The fact that an identifier is a preferred one for an organisation can be expressed by using the property E1 CRM Entity. P48 has preferred identifier (is preferred identifier of): E42 Identifier. It can better be expressed in a context independent form by assigning a suitable E55 Type, such as “preferred identifier assignment”, to the respective instance of E15 Identifier Assignment via the P2 has type property.

• replacement of the inventory number TA959a by GE34604 for a 17^th century lament cloth at the Museum Benaki, Athens
• assigning the author-uniform title heading “Goethe, Johann Wolfgang von, 1749-1832. Faust. 1. Theil.” for the respective work
• on June 1, 2001 assigning the personal name heading “Guillaume, de Machaut, ca. 1300-1377” to Guillaume de Machaut (Kelly, 2014)

• E15(x) ⇒ E13(x)

This class comprises actions measuring quantitative physical properties and other values that can be determined by a systematic, objective procedure of direct observation of particular states of physical reality.

An instance of E16 Measurement may us simple counting or tools, such as yardsticks or radiation detection devices. The interest is in the method and care applied, so that the reliability of the result may be judged at a later stage, or research continued on the associated documents. The date of the event is important for dimensions, which may change value over time, such as the length of an object subject to shrinkage. Methods and devices employed should be associated with instances of E16 Measurement by properties such as P33 used specific technique: E29 Design or Procedure, P125 used object of type: E55 Type, P16 used specific object (was used for): E70 Thing, whereas basic techniques such as "carbon 14 dating" should be encoded using P2 has type (is type of): E55 Type. Details of methods and devices reused or reusable in other instances of E16 Measurement should be documented for these entities rather than the measurements themselves, whereas details of particular execution may be documented by free text or by instantiating adequate sub-activities, if the detail may be of interest for an overarching query.

Regardless whether a measurement is made by an instrument or by human senses, it represents the initial transition from physical reality to information without any other documented information object in between within the reasoning chain that would represent the result of the interaction of the observer or device with reality. Therefore, determining properties of an instance of E90 Symbolic Object is regarded as an instance of E13 Attribute Assignment, which may be inferred from observing and measuring representative carriers. In the case that the carrier can be named, the property P16 used specific object (was used for): should be used to indicate the instance(s) of E18 Physical Thing that was used as the empirical basis for the attribute assignment. For instance, inferring properties of depicted items using image material, such as satellite images, is not regarded as an instance of E16 Measurement, but as a subsequent instance of E13 Attribute Assignment. Rather, only the production of the images, understood as arrays of radiation intensities, is regarded as an instance of E16 Measurement. The same reasoning holds for other sensor data.

• measurement of the height of silver cup 232 on the 31^st August 1997 (fictitious)
• the carbon 14 dating of the “Schoeninger Speer II” in 1996 [The carbon 14 dating of an approximately 400.000 year old complete Old Palaeolithic wooden spear found in Schoeningen, Niedersachsen, Germany, in 1995.] (Kouwenhoven, 1997)

• E16(x) ⇒ E13(x)

This class comprises the actions of classifying items of whatever kind. Such items include objects, specimens, people, actions and concepts.

This class allows for the documentation of the context of classification acts in cases where the value of the classification depends on the personal opinion of the classifier, and the date that the classification was made. This class also encompasses the notion of "determination," i.e., the systematic and molecular identification of a specimen in biology.

• the first classification of object GE34604 as Lament Cloth at the Museum Benaki, Athens
• the determination of a cactus in Martin Doerr’s garden as ‘Cereus hildmannianus K.Schumann’, July 2003

• E17(x) ⇒ E13(x)

This class comprises all persistent physical items with a relatively stable form, human-made or natural.

Depending on the existence of natural boundaries of such things, the CIDOC CRM distinguishes the instances of E19 Physical Object from instances of E26 Physical Feature, such as holes, rivers, pieces of land etc. Most instances of E19 Physical Object can be moved (if not too heavy), whereas features are integral to the surrounding matter.

An instance of E18 Physical Thing occupies not only a particular geometric space at any instant of its existence, but in the course of its existence it also forms a trajectory through spacetime, which occupies a real, that is phenomenal, volume in spacetime. We include in the occupied space the space filled by the matter of the physical thing and all its inner spaces, such as the interior of a box. For the purpose of more detailed descriptions of the presence of an instance of E18 Physical Thing in space and time it can be associated with its specific instance of E92 Spacetime Volume by the property P196 defines (is defined by).

The CIDOC CRM is generally not concerned with amounts of matter in fluid or gaseous states, as long as they are not confined in an identifiable way for an identifiable minimal time-span.

• the Cullinan Diamond (E19) (Scarratt and Shor, 2006)
• the cave “Ideon Andron” in Crete (E26) (Smith, 1844-49)
• the Mona Lisa (E22) (Mohen, 2006)

• E18(x) ⇒ E72(x)

This class comprises items of a material nature that are units for documentation and have physical boundaries that separate them completely in an objective way from other objects.

The class also includes all aggregates of objects made for functional purposes of whatever kind, independent of physical coherence, such as a set of chessmen. Typically, instances of E19 Physical Object can be moved (if not too heavy).

In some contexts, such objects, except for aggregates, are also called “bona fide objects” (Smith & Varzi, 2000, pp.401-420), i.e., naturally defined objects.

The decision as to what is documented as a complete item, rather than by its parts or components, may be a purely administrative decision or may be a result of the order in which the item was acquired.

• Aphrodite of Milos (E22) (Kousser, 2005)
• the Cullinan Diamond (Scarratt and Shor, 2006)
• Apollo 13 at the time of launch (E22) (Lovell and Kluger, 1994)

• E19(x) ⇒ E18(x)

This class comprises individual items of a material nature, which live, have lived or are natural products of or from living organisms.

Artificial objects that incorporate biological elements, such as Victorian butterfly frames, can be documented as both instances of E20 Biological Object and E22 Human-Made Object.

• me (fictitious)
• Tut-Ankh-Amun (Edwards and Boltin, 1979)
• Boukephalus [Horse of Alexander the Great] (Lamb, 2005)
• petrified dinosaur excrement PA1906-344

• E20(x) ⇒ E19(x)

This class comprises real persons who live or are assumed to have lived.

Legendary figures that may have existed, such as Ulysses and King Arthur, fall into this class if the documentation refers to them as historical figures. In cases where doubt exists as to whether several persons are in fact identical, multiple instances can be created and linked to indicate their relationship. The CIDOC CRM does not propose a specific form to support reasoning about possible identity.

In a bibliographic context, a name presented following the conventions usually employed for personal names will be assumed to correspond to an actual real person (an instance of E21 Person), unless evidence is available to indicate that this is not the case. The fact that a persona may erroneously be classified as an instance of E21 Person does not imply that the concept comprises personae.

• Tut-Ankh-Amun (Edwards and Boltin, 1979)
• Nelson Mandela (Brown and Hort, 2006)

• E21(x) ⇒ E20(x)
• E21(x) ⇒ E39(x)

This class comprises all persistent physical objects of any size that are purposely created by human activity and have physical boundaries that separate them completely in an objective way from other objects.

The class also includes all aggregates of objects made for functional purposes of whatever kind, independent of physical coherence, such as a set of chessmen.

• the Rosetta Stone (E22)
• LNER Class A4 4468 Mallard [the World’s fastest steam locomotive, preserved at the National Railway Museum of York, UK] (Solomon, 2003)
• the Portland Vase (Walker, 2004)

• E22(x) ⇒ E19(x)
• E22(x) ⇒ E24(x)

This class comprises all persistent physical items of any size that are purposely created by human activity. This class comprises, besides others, Human-Made objects, such as a sword, and Human-Made features, such as rock art. For example, a “cup and ring” carving on bedrock is regarded as instance of E24 Physical Human-Made Thing.

Instances of Human-Made thing may be the result of modifying pre-existing physical things, preserving larger parts or most of the original matter and structure, which poses the question if they are new or even Human-Made, the respective interventions of production made on such original material should be obvious and sufficient to regard that the product has a new, distinct identity and intended function and is human-made. Substantial continuity of the previous matter and structure in the new product can be documented by describing the production process also as an instance of E81 Transformation.

Whereas interventions of conservation and repair are not regarded to produce a new Human-Made thing, the results of preparation of natural history specimens that substantially change their natural or original state should be regarded as physical Human-Made things, including the uncovering of petrified biological features from a solid piece of stone. On the other side, scribbling a museum number on a natural object should not be regarded to make it Human-Made. This notwithstanding, parts, sections, segments, or features of a physical Human-Made thing may continue to be non-Human-Made and preserved during the production process, for example natural pearls used as a part of an eardrop.

• the Forth Railway Bridge (Shipway, Bouch, Baker and Fowler, 1990).
• the Channel Tunnel (E25) (Holliday, Marcou and Vickerman, 1991)
• the Palace of Knossos (Evans, 1921)
• the Coliseum in Rome, Italy (Hopkins and Beard, 2011)
• the Historical Collection of the Museum Benaki in Athens (E78) (Georgoula, 2005)
• the Rosetta Stone (E22)
• my paperback copy of Crime & Punishment (E22) (fictitious)
• the computer disk at ICS-FORTH that stores the canonical Definition of the CIDOC CRM v.3.2 (E22)
• my empty DVD disk (E22) (fictitious)

• E24(x) ⇒ E18(x)
• E24(x) ⇒ E71(x)

This class comprises physical features that are purposely created by human activity, such as scratches, artificial caves, artificial water channels, etc. In particular, it includes the information encoding features on mechanical or digital carriers.

• the Manchester Ship Canal (Farnie, 1980)
• Michael Jackson’s nose following plastic surgery
• the laser-readable “pits” engraved June 2014 on Martin Doerr’s CD-R, copying songs of Edith Piaf.
• the carved letters on the Rosetta Stone

• E25(x) ⇒ E24(x)
• E25(x) ⇒ E26(x)

This class comprises identifiable features that are physically attached in an integral way to particular physical objects.

Instances of E26 Physical Feature share many of the attributes of instances of E19 Physical Object. They may have a one-, two- or three-dimensional geometric extent, but there are no natural borders that separate them completely in an objective way from the carrier objects. For example, a doorway is a feature but the door itself, being attached by hinges, is not.

Instances of E26 Physical Feature can be features in a narrower sense, such as scratches, holes, reliefs, surface colours, reflection zones in an opal crystal or a density change in a piece of wood. In the wider sense, they are portions of particular objects with partially imaginary borders, such as the core of the Earth, an area of property on the surface of the Earth, a landscape or the head of a contiguous marble statue. They can be measured and dated, and it is sometimes possible to state who or what is or was responsible for them. They cannot be separated from the carrier object, but a segment of the carrier object may be identified (or sometimes removed) carrying the complete feature.

This definition coincides with the definition of "fiat objects" (Smith & Varzi, 2000, pp.401-420), with the exception of aggregates of “bona fide objects”.

• the cave of Dirou, Mani, Greece (Psimenos. 2005)
• the temple in Abu Simbel before its removal, which was carved out of solid rock (E25) (Hawass, 2000)
• Albrecht Duerer's signature on his painting of Charles the Great (E25) (Strauss, 1974)
• the damage to the nose of the Great Sphinx in Giza (Temple, 2009)
• Michael Jackson’s nose prior to plastic surgery

• E26(x) ⇒ E18(x)

This class comprises pieces of land or sea floor.

In contrast to the purely geometric notion of E53 Place, this class describes constellations of matter on the surface of the Earth or other celestial body, which can be represented by photographs, paintings and maps.

Instances of E27 Site are composed of relatively immobile material items and features in a particular configuration at a particular location.

• the Amazon river basin (Hegen, 1966)
• Knossos (Evans, 1921)
• the Apollo 11 landing site (Siegler and Smrekar, 2014)
• Heathrow Airport (Wicks, 2014)
• the submerged harbour of the Minoan settlement of Gournia, Crete (Watrous, 2012)
• the island of Crete

• E27(x) ⇒ E26(x)

This class comprises non-material products of our minds and other human produced data that have become objects of a discourse about their identity, circumstances of creation or historical implication. The production of such information may have been supported by the use of technical devices such as cameras or computers.

Characteristically, instances of this class are created, invented or thought by someone, and then may be documented or communicated between persons. Instances of E28 Conceptual Object have the ability to exist on more than one particular carrier at the same time, such as paper, electronic signals, marks, audio media, paintings, photos, human memories, etc.

They cannot be destroyed. They exist as long as they can be found on at least one carrier or in at least one human memory. Their existence ends when the last carrier and the last memory are lost.

• Beethoven’s “Ode an die Freude” (Ode to Joy) (E73) (Kershaw, 1999)
• the definition of “ontology” in the Oxford English Dictionary (E73) (Oxford University Press, 1989)
• the knowledge about the victory at Marathon carried by the famous runner (E89) (Lagos & Karyanos, 2020)
  • [Explanation note: In the following examples we illustrate the distinction between a propositional object, its names and its encoded forms. The Maxwell equations (Ball, 1962) are a good example, because they belong to the fundamental laws of physics and their mathematical content yields identical, unambiguous results regardless formulation and encoding.]
• “Maxwell equations” (E41) [preferred subject access point from LCSH, http://lccn.loc.gov/sh85082387, accessed 18^th April 2021. This is only the name for the Maxwell equations as standardized by the Library of Congress and not the equations themselves.]
• “Equations, Maxwell” (E41) [variant subject access point from LCSH, http://lccn.loc.gov/sh85082387, accessed 18^th April 2021. This is another name for the equation standardized by the Library of Congress and not the equations themselves.]
• Maxwell's equations (E89) [This is the propositional content of the equations proper, independent of any particular notation or mathematical formalism.] (Ball, 1962)
• The encoding of Maxwells equations as in https://upload.wikimedia.org/wikipedia/commons/c/c4/Maxwell%27sEquations.svg (E73) [accessed 18^th April 2021. This is one possible symbolic encoding of the propositional content of the equations.]

• E28(x) ⇒ E71(x)

This class comprises documented plans for the execution of actions in order to achieve a result of a specific quality, form or contents. In particular, it comprises plans for deliberate human activities that may result in new instances of E71 Human-Made Thing or for shaping or guiding the execution of an instance of E7 Activity.

Instances of E29 Design or Procedure can be structured in parts and sequences or depend on others.

This is modelled using P69 has association with (is associated with): E29 Design or Procedure.

Designs or procedures can be seen as one of the following

1. A schema for the activities it describes
2. A schema of the products that result from their application.
3. An independent intellectual product that may have never been applied, such as Leonardo da Vinci’s famous plans for flying machines.

Because designs or procedures may never be applied or only partially executed, the CIDOC CRM models a loose relationship between the plan and the respective product.

• the ISO standardisation procedure
• the musical notation for Beethoven’s “Ode to Joy” (Kershaw, 1999)
• the architectural drawings for the Kölner Dom (Cologne Cathedral) in Cologne, Germany (Wolff, 1999)
• the drawing on the folio 860 of the Codex Atlanticus from Leonardo da Vinci, 1486-1490, kept in the Biblioteca Ambrosiana in Milan

• E29(x) ⇒ E73(x)

This class comprises legal privileges concerning material and immaterial things or their derivatives.

These include reproduction and property rights.

• copyright held by ISO on ISO/CD 21127
• ownership of the “Mona Lisa” by the museum of the Louvre, Paris, France

• E30(x) ⇒ E89(x)

This class comprises identifiable immaterial items that make propositions about reality.

These propositions may be expressed in text, graphics, images, audiograms, videograms or by other similar means. Documentation databases are regarded as instances of E31 Document. This class should not be confused with the concept “document” in Information Technology, which is compatible with E73 Information Object.

• the Encyclopaedia Britannica (E32) (Kogan, 1958)
• the image content of the photo of the Allied Leaders at Yalta published by UPI, 1945 (E36)
• Domesday Book [a manuscript record of the "Great Survey" of much of England and parts of Wales completed in 1086 by order of King William the Conqueror] (Hallam 1986)

• E31(x) ⇒ E73(x)

This class comprises encyclopaedia, thesauri, authority lists and other documents that define terminology or conceptual systems for consistent use.

• Webster's Dictionary (Herbert, 1994)
• Getty Art and Architecture Thesaurus (Getty Trust, 1990)
• the CIDOC Conceptual Reference Model (Gergatsoulis et al., 2010)

• E32(x) ⇒ E31(x)

This class comprises identifiable expressions in natural language or languages.

Instances of E33 Linguistic Object can be expressed in many ways: e.g., as written texts, recorded speech or sign language. However, the CIDOC CRM treats instances of E33 Linguistic Object independently from the medium or method by which they are expressed. Expressions in formal languages, such as computer code or mathematical formulae, are not treated as instances of E33 Linguistic Object by the CIDOC CRM. These should be modelled as instances of E73 Information Object.

In general, an instance of E33 Linguistic Object may also contain non-linguistic information, often of artistic or aesthetic value. Only in cases in which the content of an instance of E33 Linguistic Object can completely be expressed by a series of binary-encoded symbols, its content may be documented within a respective knowledge base by the property P190 has symbolic content: E62 String. Otherwise, it should be understood as an identifiable digital resource only available independently from the respective knowledge base.

In other cases, such as pages of an illuminated manuscript or recordings containing speech in a language supported by a writing system, the linguistic part of the content of an instance of E33 Linguistic Object may be documented within a respective knowledge base in a note by P3 has note: E62 String. Otherwise, it may be described using the property P165 incorporates (is incorporated in): E73 Information Object as a different object with its own identity.

• the text of the Ellesmere Chaucer manuscript (Hilmo, 2019)
• the lyrics of the song "Blue Suede Shoes" (Cooper, 2008)
• the text of the “Jabberwocky” by Lewis Carroll (Carroll, 1981)
• the text of "Doktoro Jekyll kaj Sinjoro Hyde" [an Esperanto translation of Dr Jekyll and Mr Hyde]. (Stevenson, Morrison and Mann, 1909)
• the free dialog in the local dialect recorded in 1958, Telemark, Norway stored on tape or.7-89.s1 (00.15:46-00:34), The Language Collection at the University Library in Bergen, Norway (as by 2020)

• E33(x) ⇒ E73(x)

This class comprises recognisable, texts attached to instances of E24 Physical Human-Made Thing.

The transcription of the text can be documented in a note by P3 has note: E62 String. The alphabet used can be documented by P2 has type: E55 Type. This class does not intend to describe the idiosyncratic characteristics of an individual physical embodiment of an inscription, but the underlying prototype. The physical embodiment is modelled in the CIDOC CRM as instances of E24 Physical Human-Made Thing.

The relationship of a physical copy of a book to the text it contains is modelled using E18 Physical Thing. P128 carries (is carried by): E33 Linguistic Object.

• “keep off the grass” [on a sign stuck in the lawn of the quad of Balliol College, Oxford, UK]
• the text published in Corpus Inscriptionum Latinarum V 895 (Mommsen, 1872)
• “Kilroy was here”

• E34(x) ⇒ E33(x)
• E34(x) ⇒ E37(x)

This class comprises textual strings that within a cultural context can be clearly identified as titles due to their form. Being a subclass of E41 Appellation, E35 Title can only be used when such a string is actually used as a title of a work, such as a text, an artwork, or a piece of music.

Titles are proper noun phrases or verbal phrases, and should not be confused with generic object names such as “chair”, “painting” or “book” (the latter are common nouns that stand for instances of E55 Type). Titles may be assigned by the creator of the work itself, or by a social group.

This class also comprises the translations of titles that are used as surrogates for the original titles in different social contexts.

• “The Merchant of Venice” (McCullough, 2005)
• “Mona Lisa” (Mohen, Menu and Mottin, 2006)
• “La Pie” (Bortolatto, 1981)
• “Lucy in the Sky with Diamonds” (Lennon, 1967)

• E35(x) ⇒ E33(x)
• E35(x) ⇒ E41(x)

This class comprises the intellectual or conceptual aspects of recognisable marks, images and other visual works.

This class does not intend to describe the idiosyncratic characteristics of an individual physical embodiment of a visual item, but the underlying prototype. For example, a mark such as the ICOM logo is generally considered to be the same logo when used on any number of publications. The size, orientation and colour may change, but the logo remains uniquely identifiable. The same is true of images that are reproduced many times. This means that visual items are independent of their physical support.

The class E36 Visual Item provides a means of identifying and linking together instances of E24 Physical Human-Made Thing that carry the same visual qualities (symbols, marks or images etc.). The property P62 depicts (is depicted by) between E24 Physical Human-Made Thing and depicted subjects (E1 CRM Entity) is a shortcut of the more fully developed path from E24 Physical Human-Made Thing through P65 shows visual item (is shown by), E36 Visual Item, P138 represents (has representation) to E1CRM Entity, which in addition captures the optical features of the depiction.

• the visual appearance of Monet’s “La Pie” (Bortolatto, 1981)
• the Coca-Cola logo (E34)
• the Chi-Rho (E37)
• the communist red star (E37)
• the surface shape of Auguste Rodin's statue "Le Penseur" [there exist more than 20 copies, even of different size. Therefore, this is a good example that it is only the common surface shape, an immaterial visual item, which justifies displaying these copies as works of Auguste Rodin. As usual practice, Rodin himself did not produce the bronze statue, but only the prototype model]

• E36(x) ⇒ E73(x)

This class comprises symbols, signs, signatures or short texts applied to instances of E24 Physical Human-Made Thing by arbitrary techniques, often in order to indicate such things as creator, owner, dedications, purpose or to communicate information generally. Instances of E37 Mark do not represent the actual image of a mark, but the abstract ideal (or archetype) as used for codification in reference documents forming cultural documentation.

This class specifically excludes features that have no semantic significance, such as scratches or tool marks. These should be documented as instances of E25 Human-Made Feature.

• Minoan double axe mark (Lowe Fri, 2011)
• ©
• ☺

• E37(x) ⇒ E36(x)

This class comprises people, either individually or in groups, who have the potential to perform intentional actions of kinds for which someone may be held responsible.

• London and Continental Railways (E74)
• the Governor of the Bank of England in 1975 (E21)
• Sir Ian McKellan (E21) (Gibson, 1986)

• E39(x) ⇒ E77(x)

This class comprises signs, either meaningful or not, or arrangements of signs following a specific syntax, that are used or can be used to refer to and identify a specific instance of some class or category within a certain context.

Instances of E41 Appellation do not identify things by their meaning, even if they happen to have one, but instead by convention, tradition, or agreement. Instances of E41 Appellation are cultural constructs; as such, they have a context, a history, and a use in time and space by some group of users. A given instance of E41 Appellation can have alternative forms, i.e., other instances of E41 Appellation that are always regarded as equivalent independent from the thing it denotes.

Different languages may use different appellations for the same thing, such as the names of major cities. Some appellations may be formulated using a valid noun phrase of a particular language. In these cases, the respective instances of E41 Appellation should also be declared as instances of E33 Linguistic Object. Then the language using the appellation can be declared with the property P72 has language: E56 Language.

Instances of E41 Appellation may be used to identify any instance of E1 CRM Entity and sometimes are characteristic for instances of more specific subclasses E1 CRM Entity, such as for instances of E52 Time-Span (for instance “dates”), E39 Actor, E53 Place or E28 Conceptual Object. Postal addresses and E-mail addresses are characteristic examples of identifiers used by services transporting things between clients.

Even numerically expressed identifiers for extents in space or time are also regarded as instances of E41 Appellation, such as Gregorian dates or spatial coordinates, even though they allow for determining some time or location by a known procedure starting from a reference point and by virtue of that fact play a double role as instances of E59 Primitive Value.

E41 Appellation should not be confused with the act of naming something. Cf. E15 Identifier Assignment

• "Martin"
• “Aquae Sulis Minerva”
• "the Merchant of Venice" (E35) (McCullough, 2005)
• "Spigelia marilandica (L.) L." [not the species, just the name] (Hershberger, Robacker and Jenkins, 2015)
• "information science" [not the science itself, but the name through which we refer to it in an English-speaking context]
• “安” [Chinese “an”, meaning “peace”]
• “6°5’29”N 45°12’13”W” [example of a spatial coordinate]
• “Black queen’s bishop 4” [chess coordinate, example of an identifier in a conceptual space (E89)]
• “19-MAR-1922” [example of date]
• “+41 22 418 5571” [example of contact point]
• "weasel@paveprime.com" [example of contact point]
• “CH-1211, Genève” [example of place appellation]
• “1-29-3 Otsuka, Bunkyo-ku, Tokyo, 121, Japan” [example of a postal address]
• “the poop deck of H.M.S Victory” [example of a section definition on a human-made object (E22)]
• “the Venus de Milo’s left buttock” [example of a section definition on a human-made object (E22)]

• E41(x) ⇒ E90(x)

This class comprises strings or codes assigned to instances of E1 CRM Entity in order to identify them uniquely and permanently within the context of one or more organisations. Such codes are often known as inventory numbers, registration codes, etc. and are typically composed of alphanumeric sequences. Postal addresses, telephone numbers, urls and e-mail addresses are characteristic examples of identifiers used by services transporting things between clients.

The class E42 Identifier is not normally used for machine-generated identifiers used for automated processing unless these are also used by human agents.

• “MM.GE.195”
• “13.45.1976”
• “OXCMS: 1997.4.1” (fictitious)
• “ISSN 0041-5278” [Identifier for “The Unesco courier (Print)”]
• “ISRC FIFIN8900186” [Identifier for : Kraft (29 min 14 s) / Magnus Lindberg, comp. ; Toimii Ensemble ; Swedish Radio symphony orchestra ; Esa-Pekka Salonen, dir.]
• Shelf mark “Res 8 P 10”
• “Guillaume de Machaut (1300?-1377)” [a controlled personal name heading that follows the French rules] (Reaney, 1974)
• “+41 22 418 5571”
• weasel@paveprime.com
• “Rue David Dufour 5, CH-1211, Genève”
• “1-29-3 Otsuka, Bunkyo-ku, Tokyo, 121, Japan”

• E42(x) ⇒ E41(x)

This class comprises abstract temporal extents, in the sense of Galilean physics, having a beginning, an end and a duration.

Instances of E52 Time-Span have no semantic connotations about phenomena happening within the temporal extent they represent. They do not convey any meaning other than a positioning on the “time-line” of chronology. The actual extent of an instance of E52 Time-Span can be approximated by properties of E52 Time-Span giving inner and outer bounds in the form of dates (instances of E61 Time Primitive). Comparing knowledge about time-spans is fundamental for chronological reasoning.

Some instances of E52 Time-Span may be defined as the actual, in principle observable, temporal extent of instances of E2 Temporal Entity via the property P4 has time-span (is time-span of): E52 Time-Span. They constitute phenomenal time-spans as defined in CRMgeo (Doerr & Hiebel 2013). Since our knowledge of history is imperfect and physical phenomena are fuzzy in nature, the extent of phenomenal time-spans can only be described in approximation. An extreme case of approximation, might, for example, define an instance of E52 Time-Span having unknown beginning, end and duration. It may, nevertheless, be associated with other descriptions by which we can infer knowledge about it, such as in relative chronologies.

Some instances of E52 may be defined precisely as representing a declaration of a temporal extent, as, for instance, done in a business contract. They constitute declarative time-spans as defined in CRMgeo (Doerr & Hiebel 2013) and can be described via the property E61 Time Primitive P170 defines time (time is defined by): E52 Time-Span.

When used as a common E52 Time-Span for two events, it will nevertheless describe them as being simultaneous, even if nothing else is known.

• 1961
• From 12-17-1993 to 12-8-1996
• 14h30 – 16h22 4^th July 1945
• 9.30 am 1.1.1999 to 2.00 pm 1.1.1999
• the time-span of the Ming Dynasty (Chan, 2011)

• E52(x) ⇒ E1(x)

This class comprises extents in space, in particular on the surface of the earth, in the pure sense of physics: independent from temporal phenomena and matter.

The instances of E53 Place are usually determined by reference to the position of “immobile” objects such as buildings, cities, mountains, rivers, or dedicated geodetic marks, but may also be determined by reference to mobile objects. A Place can be determined by combining a frame of reference and a location with respect to this frame.

It is sometimes argued that instances of E53 Place are best identified by global coordinates or absolute reference systems. However, relative references are often more relevant in the context of cultural documentation and tend to be more precise. In particular, we are often interested in position in relation to large, mobile objects, such as ships. For example, the Place at which Nelson died is known with reference to a large mobile object – H.M.S Victory. A resolution of this Place in terms of absolute coordinates would require knowledge of the movements of the vessel and the precise time of death, either of which may be revised, and the result would lack historical and cultural relevance.

Any instance of E18 Physical Thing can serve as a frame of reference for an instance of E53 Place. This may be documented using the property P157 is at rest relative to (provides reference space for).

• the extent of the UK in the year 2003
• the position of the hallmark on the inside of my wedding ring (fictitious)
• the place referred to in the phrase: “Fish collected at three miles north of the confluence of the Arve and the Rhone”
• here -> <- [the place between these two arrows in one of the reader's paper copy of this document. Each copy constitutes a different place of this spot.]

• E53(x) ⇒ E1(x)

This class comprises quantifiable properties that can be measured by some calibrated means and can be approximated by values, i.e., by points or regions in a mathematical or conceptual space, such as natural or real numbers, RGB values etc.

An instance of E54 Dimension represents the empirical or theoretically derived quantity, including the precision tolerances resulting from the particular method or calculation. The identity of an instance of E54 Dimension depends on the method of its determination because each method may produce different values even when determining comparable qualities. For instance, the wingspan of a bird alive or dead is a different dimension. Thermoluninescence dating and Rehydroxylation [RHX] dating are different dimensions of temporal distance from now, even if they aim at dating the same object. The method of determination should be expressed using the property P2 has type (is type of). Note that simple terms such as “diameter” or “length” are normally insufficient to unambiguously describe a respective dimension. In contrast, “maximum linear extent” may be sufficient.

The properties of the class E54 Dimension allow for expressing the numerical approximation of the values of instances of E54 Dimension adequate to the precision of the applied method of determination. If the respective quantity belongs to a non-discrete space according to the laws of physics, such as spatial distances, it is recommended to record them as approximations by intervals or regions of indeterminacy enclosing the assumed true values. For instance, a length of 5 cm may be recorded as 4.5-5.5 cm, according to the precision of the respective observation. Note, that comparability of values described in different units depends critically on the representation as value regions.

Numerical approximations in archaic instances of E58 Measurement Unit used in historical records should be preserved. Equivalents corresponding to current knowledge should be recorded as additional instances of E54 Dimension, as appropriate.

• the weight of the Luxor Obelisk [250 metric tons]
• the vertical height of the statue of David by Michaelangelo [5.17 metres]
• the weight of the Great Star of Africa diamond [530.2 carats]
• the calibrated C14 date for the Shroud of Turin [AD1262-1312, 1303-1384]
• the horizontal diameter of the Stonehenge Sarsen Circle [33 metres] (Pryor, 2016)
• the length of the sides of the Great Pyramid at Giza [230.34 metres] (Lehner and Hawass, 2017)
• the duration of the time span of the Battle of Issos/Issus on 15^th November 333 B.C.E. [less than 12 hours] (Howard, 2012)
• Christie’s hammer price, in British Pounds, for Vincent van Gogh's "Still Life: Vase with Fifteen Sunflowers" in London on 30^th March 1987 (E97) [24.75 million GBP (Brithish Pounds)]

• E54(x) ⇒ E1(x)

This class comprises concepts denoted by terms from thesauri and controlled vocabularies used to characterize and classify instances of CIDOC CRM classes. Instances of E55 Type represent concepts in contrast to instances of E41 Appellation which are used to name instances of CIDOC CRM classes.

E55 Type is the CIDOC CRM’s interface to domain specific ontologies and thesauri. These can be represented in the CIDOC CRM as subclasses of E55 Type, forming hierarchies of terms, i.e., instances of E55 Type linked via P127 has broader term (has narrower term): E55 Type. Such hierarchies may be extended with additional properties.

• weight, length, depth [types for instances of E54]
• portrait, sketch, animation [types for instances of E36]
• French, English, German (E56)
• excellent, good, poor [types for instances of E3]
• Ford Model T, chop stick [types for instances of E22]
• cave, doline, scratch [types for instances of E26]
• poem, short story [types for instances of E33]
• wedding, earthquake, skirmish [types for instances of E5]

• E55(x) ⇒ E28(x)

This class is a specialization of E55 Type and comprises the natural languages in the sense of concepts.

This type is used categorically in the model without reference to instances of it, i.e., the Model does not foresee the description of instances of instances of E56 Language, e.g.: “instances of Mandarin Chinese”.

It is recommended that internationally or nationally agreed codes and terminology are used to denote instances of E56 Language, such as those defined in ISO 639-1:2002 and later versions.

• el [Greek] (Palmer, 1980)
• en [English] (Wilson, 1983)
• eo [Esperanto] (Nuessel, 2000)
• es [Spanish] (Pineda, 1993)
• fr [French] (Rickard, 1974)

• E56(x) ⇒ E55(x)

This class is a specialization of E55 Type and comprises the concepts of materials.

Instances of E57 Material may denote properties of matter before its use, during its use, and as incorporated in an object, such as ultramarine powder, tempera paste, reinforced concrete. Discrete pieces of raw-materials kept in museums, such as bricks, sheets of fabric, pieces of metal, should be modelled individually in the same way as other objects. Discrete used or processed pieces, such as the stones from Nefer Titi's temple, should be modelled as parts (cf. P46 is composed of (forms part of): E18 Physical Thing).

This type is used categorically in the model without reference to instances of it, i.e., the Model does not foresee the description of instances of instances of E57 Material, e.g.: “instances of gold”.

It is recommended that internationally or nationally agreed codes and terminology are used.

• brick (Gurcke, 1987)
• gold (Watson, 1990)
• aluminium (Norman, 1986)
• polycarbonate (Mhaske, 2011)
• resin (Barton, 1992)

• E57(x) ⇒ E55(x)

This class is a specialization of E55 Type and comprises the types of measurement units: feet, inches, centimetres, litres, lumens, etc.

This type is used categorically in the model without reference to instances of it, i.e., the Model does not foresee the description of instances of instances of E58 Measurement Unit, e.g.: “instances of cm”.

Système International (SI) units or internationally recognized non-SI terms should be used whenever possible, such as those defined by ISO80000:2009. Archaic Measurement Units used in historical records should be preserved.

• cm [centimetre]
• km [kilometre]
• m [meter]
• m/s [meters per second] (Hau et al., 1999)
• A [Ampere]
• GRD [Greek Drachme] (E98) (Daniel, 2014)
• C [degrees centigrade] (Beckman, 1998)

• E58(x) ⇒ E55(x)

This class comprises values of primitive data types of programming languages or database management systems and data types composed of such values used as documentation elements, as well as their mathematical abstractions.

The instances of E59 Primitive Value and its subclasses are not considered elements of the universe of discourse the CIDOC CRM aims to define and analyze. Rather, they play the role of a symbolic interface between the scope of the model and the world of mathematical and computational manipulations and the symbolic objects they define and handle.

In particular, they comprise lexical forms encoded as "strings" or series of characters and symbols based on encoding schemes (characterised by being a limited subset of the respective mathematical abstractions) such as UNICODE and values of datatypes that can be encoded in a lexical form, including quantitative specifications of time-spans and geometry. They have in common that instances of E59 Primitive Value define themselves by virtue of their encoded value, regardless of the nature of their mathematical abstractions.

Therefore, in an implementation, instances of E59 Primitive should be represented directly in the encoded symbolic form supported by the respective platform, such as a character string or a formatted date. They must not be represented in an implementation indirectly via, another a universal resource identifier, which in turn is linked to the actual encoded symbolic form. In a concrete application, it is recommended that the primitive value system from a chosen implementation platform and/or data definition language be used to substitute for this class and its subclasses.

• “ABCDEFG” (E62)
• 3.14 (E60)
• 0 (E60)
• 1921-01-01 (E61)

• E59(x) ⇒ E1(x)

This class comprises any encoding of computable (algebraic) values such as integers, real numbers, complex numbers, vectors, tensors etc., including intervals of these values to express limited precision.

Numbers are fundamentally distinct from numerically expressed identifiers in continua, which are instances of E41 Appellation, such as Gregorian dates or spatial coordinates, even though their encoding may be similar. Instances of E60 Number can be combined with each other in algebraic operations to yield other instances of E60 Number, e.g., 1+1=2. Identifiers in continua may be combined with numbers expressing distances to yield new identifiers, e.g., 1924-01-31 + 2 days = 1924-02-02. Cf. E54 Dimension.

• 5
• 3+2i
• 1.5e-04
• (0.5, - 0.7,88)

• E60(x) ⇒ E59(x)

This class comprises instances of E59 Primitive Value for time that should be implemented with appropriate validation, precision and references to temporal coordinate systems to express time in some context relevant to cultural and scientific documentation.

Instantiating different instances of E61 Time Primitive relative to the same instance of E52 Time Span allows for the expression of multiple opinions/approximations of the same phenomenon. When representing different opinions/approximations of the E52 Time Span of some E2 Temporal Entity, multiple instances of E61 Time Primitive should be instantiated relative to one E52 Time Span. Only one E52 Time Span should be instantiated since there is only one real phenomenal time extent of any given temporal entity.

The instances of E61 Time Primitive are not considered as elements of the universe of discourse that the CIDOC CRM aims at defining and analysing. Rather, they play the role of a symbolic interface between the scope of this model and the world of mathematical and computational manipulations and the symbolic objects they define and handle.

Therefore, they must not be represented in an implementation by a universal identifier associated with a content model of different identity. In a concrete application, it is recommended that the primitive value system from a chosen implementation platform and/or data definition language be used to substitute for this class.

• “1994 – 1997”
• “13^th May 1768”
• “2000/01/01 00:00:59.7”
• “85^th century BCE”

• E61(x) ⇒ E41(x)
• E61(x) ⇒ E59(x)

This class comprises coherent sequences of binary-encoded symbols. They correspond to the content of an instance of E90 Symbolic object. Instances of E62 String represent only the symbol sequence itself. They may or may not contain a language code.

In contrast, instances of other subclasses of E59 Primitive value represent entities in mathematical spaces other than that of symbol sequences, by using binary-encoded symbols, such as date expressions or numbers in decimal encoding. For instance, different syntactic forms of a date expression may represent the same date but consist of different strings.

• “the Quick Brown Fox Jumps Over the Lazy Dog”
• “6F 6E 54 79 70 31 0D 9E”

• E62(x) ⇒ E59(x)

This class comprises events that bring into existence any instance of E77 Persistent Item.

It may be used for temporal reasoning about things (intellectual products, physical items, groups of people, living beings) beginning to exist; it serves as a hook for determination of a “terminus post quem” or “terminus ante quem”.

• the birth of my child (E67) (fictitious)
• the birth of Snoopy, my dog (fictitious)
• the calving of the iceberg that sank the Titanic
• the construction of the Eiffel Tower (E12) (Tissandier, 1889)

• E63(x) ⇒ E5(x)

This class comprises events that end the existence of any instance of E77 Persistent Item.

It may be used for temporal reasoning about things (physical items, groups of people, living beings) ceasing to exist; it serves as a hook for determination of a “terminus post quem” or “terminus ante quem”. In cases where substance from an instance of E64 Persistent Item continues to exist in a new form, the process would be documented as instances of E81 Transformation.