The Science of Subjects
Library of Congress Subject Headings
In the example on the previous page, "pet therapy" is considered natural language, searched as a keyword. The phrase "Pets - Therapeutic Use" used in the subject field is called controlled vocabulary.
Controlled vocabulary terms in the subject field all come from a set of books called the Library of Congress Subjects Headings, or LCSH. The LCSH (sometimes called the "red books") usually hang out near the reference desk.