Three popular knowledge organization systems (KOSs)—the Encyclopædia Britannica’s “Outline of Knowledge,” Roget’s International Thesaurus’s “Synopsis of Categories,” and the Dewey Decimal Classification—are compared in the context of a taxonomy of evaluation methods for KOSs that takes into account similarities and differences in formats and purposes. The goals are to argue for the wider adoption of a framework for KOS evaluation of the kind presented here; to promote the treatment of encyclopedia outlines and thesaurus synopses as well as library classification schemes as KOSs assessable via such a framework; and to improve our understanding of KOSs in general.