allow class enumeration, cardinality, disjointness, and equivalence. Metaclasses Metaclasses are classes of classes. They are allowed in OWL full or with a feature called class/instance punning. Public ontologies Libraries Biomedical OBO Foundry NCBO BioPortal NCI Enterprise Vocabulary Services Standards Suggested Upper Merged Ontology (SUMO) TDWG PROV-O, the ontology version of the W3C's PROV-DM Basic Formal Ontology (BFO) European Materials Modelling Ontology (EMMO) Browsers The following tools include public ontology browsers: Protégé OWL Search Swoogle Limitations No direct language support for n-ary relationships. For example, modelers may wish to describe the qualities of a relation, to relate more than 2 individuals or to relate an individual to a list. This cannot be done within OWL. They may need to adopt a pattern instead which encodes the meaning outside the formal semantics. See also RDF Semantic technology Agris: International Information System for the Agricultural Sciences and Technology Common Logic FOAF + DOAC Frame language Geopolitical ontology IDEAS Group Meta-Object Facility (MOF), a different standard for the Unified Modeling Language (UML) of the Object Management Group (OMG) Metaclass (Semantic Web), a featured allowed by OWL to represent knowledge Multimedia Web Ontology Language Semantic reasoner SKOS SSWAP: Simple Semantic Web Architecture and Protocol SHACL: Shapes and Constraints Language for RDF References Further reading World Wide Web Consortium standards Resource Description Framework XML-based standards Declarative programming languages Ontology languages Semantic Web