RT is a convention used to indicate a relationship between terms that are not associative or hierarchical. RT follows a term and precedes a related term. Use of RT can vary based on the informational retrieval system organization. Generally, related terms in an information retrieval system are terms that are overlapping sibling terms, mutually exclusive sibling terms, terms in derivational relationships, or terms in different hierarchies.  For example, "boats RT ships" indicated that "boats" is a related term to "ships."

San Jose State University

A cross-reference that indicates a relationship between terms that are related. An associative relationship is established to outline subject areas in a given domain and to aid users in browsing.

Associative Cross-Reference Relationships 

ANSI/NISO. (2005). Guidelines for the Construction, Format, and Management of monolingual Controlled Vocabularies. American National Standards Institute. https://groups.niso.org/higherlogic/ws/public/download/12591/z39-19-2005r2010.pdf 

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