A thesaurus has equivalence (USE/UF), broader term (BT), narrower term (NT) and related term (RT) relationships. These relationships provide structure for the terms. For instance, knowing that a broader term for "cereals" is "plant products" and that narrower terms are "maize" and "rye" provides a structure that defines the scope of those terms.
Recently, there has been considerable discussion relating to extending this core set of relationships. In the late 1990s, the American Library Association Subcommittee on Subject Relationships/Reference Structures examined over 165 relationships within the English language alone and from these produced a checklist of twenty candidate subject relationships for information retrieval.
