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."