The document discusses the Sears List of Subject Headings (SLSH), a controlled vocabulary used for subject cataloging in small to medium sized libraries. It provides an overview of the history and purpose of SLSH, describes some of its key features like new subject headings added in the 21st edition, and outlines its underlying principles of direct, specific, and consistent subject entries based on common usage. The structure of SLSH is also briefly explained as an alphabetical list of subject terms for the entire range of knowledge.