Categories are considered to be an essential element in the design of many knowledge organization systems (KOS), particularly those following the categorial approach of faceted analysis. Standard narratives of knowledge organization (KO) identify S.R. Ranganthan as the founder of the categorial approach to KO. However, they also acknowledge that elements of a categorial approach to KO can be found in a number of earlier KOSs created in the late 19th and early 20th centuries by such pioneers of KO as Paul Otlet, James D. Brown, and Julius O. Kaiser. This paper seeks to expand the list of precursors to the categorial approach by examining the rôle of categories in two KOSs created by Charles Ammi Cutter: the system of subject cataloging outlined in the Rules for a Dictionary Catalog (RDC) and the bibliographical classification known as the Expansive Classification (EC). In the RDC, Cutter divided subjects into three categories—concrete individual, concrete general, and abstract general—which were articulated into a classificatory, hierarchical structure that provided the framework for his principle of specific entry. Cutter also established a “significance order” of precedence for these categories, which functioned as a decision tree for selecting the most specific subject headings under which books treating of complex subjects should be entered. In the EC, Cutter divided the classification into schedules for subjects, a list of bibliographical forms, and an auxiliary table of place names (the Local List), with each of these divisions sharply distinguished from the other by notational means; moreover, he established a mechanism for class synthesis, whereby classes in the classification could take the forms Subject–Place or Place–Subject. Both KOSs were designed to allow maximal treatment of concrete, specific classes and so manifest the underlying unity of Cutter’s vision of KO. The pervasive, but implicit and untheorized, use of categorial structures in both the RDC and EC justifies the inclusion of Cutter among the precursors of the categorial approach to KO.