Intended to facilitate the preparation of thesauri for the indexing of documents in any subject field, this report presents a set of rules pertaining to the complex issues and questions involved in indexing vocabulary development and construction. After a brief discussion of the nature, functions, and components of indexing vocabularies, a set of definitions of terms and a list of abbreviations used in the paper are presented. Vocabulary control, various forms of indexing terms, the use of singular and plural forms in indexing languages, the choice of appropriate terminology, proper names in indexing languages, and the functions of scope notes and definitions are discussed. Compound index terms are then analyzed in detail and the basic semantic relationships in indexing languages are described, with attention given to equivalence, hierarchical, associative, and inter-category relationships. The display of terms and their relationships is also reviewed and management aspects of thesaurus construction are considered, including methods of compilation, admission and deletion of terms, automated data processing, and editorial issues. An explanation of symbols used to express thesaural relationships is appended. (JL)