The thesaurus is only one of a variety of tools that are used to index or tag documents for the purpose of information storage and retrieval. The term ‘thesaurus’ is often applied fairly loosely to a number of these, with the general sense of some kind of a subject-related vocabulary. In this chapter I shall try to identify the main types of vocabulary tool which you may come across, and to determine their significant characteristics. Despite the existence of published standards for many of these tools, in practice the terminology is not applied very precisely, and it is easy to be confused by the different understanding of these names. We have already mentioned classification schemes and subject heading lists used in conventional library and document collections, and keyword lists used for post-coordinate indexing, as well as the thesaurus proper. More recently conceived types of subject tool include the taxonomy, the concept map, and the ontology. These sorts of system are sometimes referred to collectively as controlled vocabularies, or controlled indexing languages, to contrast them with the use of natural, or uncontrolled, language in subject indexing. They may also be described as knowledge organization systems, or knowledge organization structures, by those whose primary interest is in the analysis and structure of subject fields or domains, and the conceptual relationships between subjects.

Like natural languages such as English, Chinese or Arabic, the indexing language has a vocabulary (the terms used for indexing) and syntax, or operating rules. The ‘control’ is imposed by the compiler of the vocabulary, and consists of limits placed on the number and form of words or terms that can be used in indexing. This enables synonyms and variant forms of words to be managed in a way that supports more efficient indexing and retrieval, and avoids overlap and confusion in the use of similar concepts. Strictly speaking, vocabulary control refers only to this process of linguistic management, but controlled vocabularies commonly exhibit other features, such as the identification of relationships between terms, and rules for combining terms when necessary (the system syntax mentioned above). The advantages of using controlled languages are discussed in Chapter 5.