b'Principles of the Sears Listof Subject HeadingsCertain principles and practices of subject cataloging should be understood before an attempt ismade to assign subject headings to library materials. The discussion that follows makes reference tothe Sears List of Subject Headings, henceforth referred to as the Sears List or the List, but theprinciples are applicable to other lists of subject headings as well.The Purpose of Subject CatalogingAll library work is a matter of the storage and retrieval of information, and cataloging is that aspectof library work devoted to storage. The best cataloging is simply that which facilitates the mostaccurate and complete retrieval. The two basic branches of cataloging are descriptive cataloging andsubject cataloging. Descriptive cataloging makes possible the retrieval of materials in a library bytitle, author, date, etc.in short all the searchable elements of a cataloging record except thesubjects. Only by conforming to the standards for descriptive cataloging can a librarian assure theuser accurate retrieval on the descriptive elements. Those standards are codified in ResourceDescription and Access (RDA), which is in the process of replacing the older Anglo-AmericanCataloguing Rules, second revised edition (AACR2).Until the second half of the nineteenth century, descriptive cataloging was the only librarycataloging that was found necessary. Libraries were much smaller than they are today, and scholarlylibrarians then were able, with the aid of printed bibliographies, to be familiar with everythingavailable on a given subject and guide the users to it. With the rapid growth of knowledge in manyfields in the course of the nineteenth century and the resulting increase in the volume of books andother library materials, it became desirable to do a preliminary subject analysis of such works andthen to represent them in the catalog in such a way that they would be retrievable by subject.Subject cataloging deals with what a book or other library item is about, and the purpose of subjectcataloging is to list under one uniform word or phrase all the materials on a given topic that a libraryhas in its collection. A subject heading is that uniform word or phrase used in the library catalog toexpress a topic. The use of authorized words or phrases only, with cross-references fromunauthorized synonyms, is the essence of bibliographic control in subject cataloging. The purposeof a subject authority, such as the Sears List, is to provide a basic vocabulary of authorized termstogether with suggestions for useful cross-references.The two most common types of subject authorities are the thesaurus and the subject heading list. Atrue thesaurus, in the realm of information science, is a comprehensive controlled vocabulary ofdiscrete unit terms, called descriptors, arranged is such a way as to display the hierarchical andassociative relationships among terms. It is usually limited to a particular realm of knowledge, as inthe case of the Art and Architecture Thesaurus. The American national standards for thesauri areA-17'