Knowledge Organization Systems (KOS), concept system or concept scheme is a generic term used in knowledge organization for authority files, classification schemes, thesauri, topic maps, ontologies and similar works. Despite their differences in type, coverage and application all KOS aim to support the organization of knowledge and information to facilitate their management and retrieval. The core elements of most KOS can be expressed in RDF with the Simple Knowledge Organization System (SKOS). Many lists of KOS exist with BARTOC being the largest and most general one.
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Networked Knowledge Organization Systems/Services NKOS: http://nkos.slis.kent.edu/