KL-ONE lets one define and use a class of descriptive terms called Concepts, where each Concept denotes a set of objects A subsumption relation between Concepts is defined which is related to set inclusion by way of a semantics for Concepts. This subsumption relation defines a partial order on Concepts, and KL-ONE organizes all Concepts into a taxonomy that reflects this partial order. Classification is a process that takes a new Concept and determines other Concepts that either subsume it or that it subsumes, thereby determining the location for the new Concept within a given taxonomy. We discuss these issues and demonstrate some uses of the classification algorithm. KL-ONE is a knowledge representation system developed at Bolt Beranek and Newman over the past few years (see [Brachman 77, Brachman 79, Schmolze 82. Sidner 81]), that grew out of semantic network formalisms. The primary unit of