The Simple Knowledge Organization System, or Skos, provides a core vocabulary for defining and organizing pieces of knowledge, known as Concepts. The model is generic enough to be usable for everything from metadata tags to thesauruses, and is particularly useful as the basis for more specialized knowledge models.

Skos expresses constraints (i·e, integrety conditions) on some of its terms which are not formally expressible in Owl. Some constraints, like those involving transitive properties, are expressible but not reasonable, and so are left out of this ontology. Constraints are only minimally useful in an ontological sense anyway, as reasoning systems make for poor validators.

Skos formally defines its simple labels as annotation properties, meaning they cannot be reasoned about. Skos·X·L (defined in an appendix to the Skos specification) instead defines labels using objects, enabling reasoning.

Skos·X·L also defines simple labels in terms of property chains of complex ones, which is practically impossible if the simple labels are truly annotation properties. This ontology chooses to resolve this tension by treating the relationship between simple and complex properties as informal.

Skos·X·L labels are generally recommended anyway, as the ability to attach additional properties to them is often useful.

This ontology includes all of Skos, as it is incredibly useful baseline model for knowledge organization and representation.