XKOS - Extended Knowledge Organization System

Published: 2019-05-01 Updated: 2022-02-17*

* No changes in the text. HTML anchors were added in the vocabulary summary table so that terms could be referenced from outside documents (see commit d59ed90).




XKOS leverages the Simple Knowledge Organization System (SKOS) for managing statistical classifications and concept management systems, since SKOS is widely used. LOD is used to create Web artifacts that machines can interpret, so publishing machine-readable statistical classifications and other concept management systems as SKOS instances is desired. The XKOS developers found that SKOS was insufficient for the problem. No aspect of SKOS was found to be wrong, just incomplete. Therefore, an extension to SKOS, called XKOS, is proposed.

XKOS extends SKOS for the needs of statistical classifications. It does so in two main directions. First, it defines a number of terms that enable the representation of statistical classifications with their structure and textual properties, as well as the relations between classifications. Second, it refines SKOS semantic properties to allow the use of more specific relations between concepts. Those specific relations can be used for the representation of classifications or for any other case where SKOS is employed. XKOS adds the extensions that are desirable to meet the requirements of the statistical community.



  • Publication of Statistical Classification in and web environment
  • Refined specification of relationships between concepts in a classification structure

Informational Documentations


Markup Examples



The XKOS vocabulary is licensed under a Creative Commons Attribution 4.0 License


Development Work

English: "XKOS: an SKOS extension for representing statistical classifications" - This ontology is based on work initiated at Dagstuhl Schloss in September 2011

French "XKOS : une extension de SKOS pour la représentation des nomenclatures statistiques" - Cette ontologie est basée sur le travail commencé à Dagstuhl Schloss en septembre 2011

Contributors: Daniel Gillman, Wendy L. Thomas, Richard Cyganiak, Jannik Jensen, Thomas Bosch, R.T.A.M. Grim, Yves Jaques


Future Work

Development of DDI XKOS is managed by the DDI XKOS Working Group.

Selected Articles