SKOS Core Vocabulary

Abstract

SKOS Core is a model for expressing the structure and content of concept schemes (thesauri, classification schemes, subject heading lists, taxonomies, terminologies, glossaries and other types of controlled vocabulary).

The SKOS Core Vocabulary is an application of the Resource Description Framework (RDF), that can be used to express a concept scheme as an RDF graph. Using RDF allows data to be linked to and/or merged with other RDF data by semantic web applications.

This document gives a reference-style overview of the SKOS Core Vocabulary as it stands at the time of publication. It also describes the policies for ownership, naming, persistence and change by which the SKOS Core Vocabulary is managed.

This edition of the SKOS Core Vocabulary Specification is a W3C Public Working Draft. It is the authoritative human-readable account of the SKOS Core Vocabulary at the time of publication.

See also the SKOS Core Guide.

Status of this Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

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Contents

• Introduction
• Guide to Term Summary Tables
• Policy Statements
  • Ownership
  • Naming
  • Persistence
  • Change
• Classes
• Properties
• Deprecated
• Release Notes
• Acknowledgements

Introduction

SKOS stands for Simple Knowledge Organisation System. The name SKOS was chosen to emphasise the goal of providing a simple yet powerful model for expressing knowledge organisation systems in a machine-understandable way, within the framework of the Semantic Web.

SKOS Core is a model for expressing the structure and content of concept schemes (thesauri, classification schemes, subject heading lists, taxonomies, terminologies, glossaries and other types of controlled vocabulary).

The SKOS Core Vocabulary is an application of the Resource Description Framework (RDF). RDF provides a simple data formalism for talking about things, their properties, inter-relationships, and categories (classes). Using RDF allows data to be linked to and/or merged with other RDF data by Semantic Web applications. In practice, this means that data sources can be distributed across the web in a decentralised way, but still be meaningfully composed and integrated by applications, often in novel and unanticipated ways. See RDF Concepts for an overview of RDF, RDF Semantics for its formal mathematical basis, and RDF Syntax for details of the RDF/XML document format used to exchange RDF data.

This document gives a reference-style overview of the SKOS Core Vocabulary as it stands at the time of publication. It also describes the policies for ownership, naming, persistence and change by which the SKOS Core Vocabulary is managed.

This edition of the SKOS Core Vocabulary Specification is a W3C Public Working Draft. It has been reviewed and approved for publication by the Semantic Web Best Practices and Deployment Working Group. It is the authoritative human-readable account of the SKOS Core Vocabulary at the time of publication.

A formal representation of the SKOS Core Vocabulary is maintained in RDF/OWL [latest: http://www.w3.org/2004/02/skos/core]. Historical snapshots of the RDF/OWL description of the SKOS Core Vocabulary can be obtained from [http://www.w3.org/2004/02/skos/core/history/].

Guide to Term Summary Tables

Each term (i.e. class or property) of the vocabulary is summarised in this document as a table. Each table may have the following rows:

Class or Property:
URI:	The Universal Resource Identifier.
Label:	A human-readable label.
Definition:	An explanation of the meaning of a class or property.
Comment:	Additional information about meaning and/or proper use.
Example:	An example of the use of a class or property.
Status:	The status (stability level) of the class or property.
Issued:	Date on which the class or property was issued.
Modified:	Date on which the class or property was last modified.
Super-classes:	(Classes only) any declared super-classes.
Super-properties:	(Properties only) any declared super-properties.
Domain:	(Properties only) the declared domain for the property.
Range:	(Properties only) the declared range for the property.
Additional types:	(Properties only) any declared additional types (e.g. owl:TransitiveProperty) for the property.
Inverse of:	(Properties only) any declared inverse properties.
Replaces:	Any deprecated terms which the given term has replaced in recommended usage.
Version info:	A note about the modification and/or history of a class or property.
Replaced By:	(Deprecated terms only) the term to use instead of the deprecated term.
Deprecated:	(Deprecated terms only) the date of last modification (i.e. deprecation) of the term.

Policy Statements

N.B. The Working Group is committed to establishing clear expectations around the management of RDF vocabularies, through documentation of process and maintenance policies. This is itself an evolving process. Specifically, this document is itself situated within the W3C Process, and may change and evolve in the light of feedback on SKOS Core and on the SKOS Core policy statements. It should be noted that claims made by the Working Group using the (experimental) persistence and change terminology employed here have as their scope the currently chartered Working Group. They have only draft status within the wider W3C Process. W3C has not delegated to the Working Group any authority to make binding commitments on behalf of W3C beyond those implicit in the formal W3C Process.

Ownership

The W3C gives control over the SKOS Core Vocabulary to working groups within the overall framework of the W3C process. Currently that control resides with the Semantic Web Best Practices and Deployment Working Group, whose chairs have delegated responsibility for maintaining the SKOS Core Vocabulary to the editors of this specification (Alistair Miles and Dan Brickley). When this working group's charter expires, control will revert to W3C as an organization.

The Working Group is committed to a public, consensus-driven design environment for SKOS Core, and to this end conducts SKOS-related discussion in public, in particular drawing on feedback from the Semantic Web Interest Group mailing list public-esw-thes@w3.org.

Naming

The URI for the SKOS Core Vocabulary itself is:

http://www.w3.org/2004/02/skos/core

The URI for a class or property in the SKOS Core Vocabulary is constructed by appending a fragment identifier to the URI for the SKOS Core Vocabulary. E.g.

        
        http://www.w3.org/2004/02/skos/core#Concept
        http://www.w3.org/2004/02/skos/core#prefSymbol
    

A fragment identifier for a class always starts with an uppercase character. Where the fragment identifier is comprised of multiple concatenated words, the leading character of each word will be an uppercase character. E.g.

        Concept
        ConceptScheme
    

The fragment identifier for a property starts with a lowercase character. Where the fragment identifier is comprised of multiple concatenated words, the leading character of the second and each subsequent word will be an uppercase character. E.g.

        subject
        prefLabel
        isPrimarySubjectOf
    

Persistence

All editions of the SKOS Core Vocabulary Specification, all editions of the SKOS Core Guide, and the RDF/OWL description of the SKOS Core Vocabulary [http://www.w3.org/2004/02/skos/core] are all declared to be persistent resources, as defined by the persistence policy at [http://www.w3.org/Consortium/Persistence].

Change

The SKOS Core Vocabulary may change. The process for managing changes to the SKOS Core Vocabulary during the chartered lifetime of the Working Group is described below:

1. The Working Group undertakes to review the SKOS Core Vocabulary Specification and the SKOS Core Guide at intervals of 2 months. Subsequent to each review, new Public Working Draft editions of the SKOS Core Guide and the SKOS Core Vocabulary Specification will be published by the Working Group.
2. In the interim period between publication of Public Working Draft editions of the SKOS Core Guide and the SKOS Core Vocabulary Specification (hereafter 'the interim period'), no changes will be made to the SKOS Core Vocabulary. The formal RDF/OWL description of the SKOS Core Vocabulary [http://www.w3.org/2004/02/skos/core] will therefore also remain unchanged during this period.
3. In the interim period, the editor's will maintain a public list of proposed changes to the SKOS Core Vocabulary to be put forward at the next review.
4. Members of the public may make proposals for change, or comment on current proposals, by sending an email to the publicly archived Semantic Web Interest Group mailing list public-esw-thes@w3.org [archive].
5. Proposed changes to be put forward for review will be published at least 2 weeks before a scheduled review, to allow time for public comment.
6. At each review, the list of proposed changes to the SKOS Core Vocabulary is presented to the reviewers delegated by the Working Group for approval.
7. Those changes approved by the reviewers, or approved in a modified form after negotiation between the editors and the reviewers, will be implemented by the editors. New Public Working Draft editions of the SKOS Core Guide and SKOS Core Vocabulary Specification, reflecting these changes, will then be published by the Working Group. Approved changes will also be implemented in the formal representation of the SKOS Core Vocabulary in RDF/OWL [http://www.w3.org/2004/02/skos/core]. Therefore the formal representation of the SKOS Core Vocabulary in RDF/OWL [http://www.w3.org/2004/02/skos/core] will always be consistent with the latest Public Working Draft editions of the SKOS Core Guide and the SKOS Core Vocabulary Specification.
8. All changes made will also be reported on the SKOS Change Log [http://esw.w3.org/mt/esw/archives/cat_skos_changelog.html].

Furthermore, at any given time only certain types of change are allowed (following the example of the DCMI namespace policy [http://dublincore.org/documents/dcmi-namespace/]). The types of change allowed depend on the 'status' of the class or property to which the change relates. The status of a class or property may take one of three values:

unstable

The term is unstable, and feedback is welcomed on its current form and utility. It may currently be poorly defined. Its meaning and/or form may be expected to change at any time. Do not implement mission critical systems that depend on this term persisting in its current form. (Changes corresponding to DCMI Namespace Policy types A, B or C may occur.)

testing

The term has gone beyond the raw proposal stage, and is undergoing testing. This term may still change in response to feedback from testing, although it may be expected not to undergo any radical change. The cost to early implementors of changing the term will be considered, however the goal of achieving wider interoperability and long-term stability may override those considerations. (Changes corresponding to DCMI Namespace Policy types A or B may occur.)

stable

No substantial (i.e. meaning-changing) alterations will take place. Implementors can expect the term to persist in its current form indefinitely. (Minor editorial changes corresponding to DCMI Namespace Policy type A may occur.)

New classes or properties may be added to the SKOS Core Vocabulary (changes corresponding to DCMI Namespace Policy type D), in accordance with the process described above.

A class or property at any of the above status levels may be marked as deprecated, in which case instructions will be given on what to use in its place. A deprecated class property may be expected to remain unchanged indefinitely.

Historical versions (snapshots) of the SKOS Core Vocabulary are maintained, and can be accessed via the web page at [http://www.w3.org/2004/02/skos/core/history/].

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Release Notes

The following changes have been made since the previous edition of the SKOS Core Vocabulary Specification:

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Acknowledgements

This work has benefited greatly from contributions made by members of the public-esw-thes@w3.org mailing list. Their feedback, suggestions and encouragement are gratefully acknowledged. SKOS Core draws heavily on conventions and traditions in the thesaurus and library community, whose practitioners have very generously shared their experiences and insights during the design of SKOS Core.

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