BT is a convention used to indicate a relationship between broader and narrower terms. BT follows a narrower term and precedes a broader term. It is used to identify broader terms than the term searched in an information retrieval system. For example "biology BT science" indicates tha "science" is a broader term than "biology."

NT is a convention used to indicate a relationship between narrower and broader terms.  NT follows a broader term and precedes a narrower term. It is used to identify narrower terms than the term searched in an information retrieval system. For example, "math NT algebra" indicated that "algebra" is a narrower term than "math." 

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A cross-reference that indicates a relationship between subordinate and superordinate terms. Subordinate terms are those that are members, parts, examples, or instances of a superordinate term.

Hierarchical Cross-Referencing 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 

Guidelines for the Construction, Format, and Management of monolingual Controlled Vocabularies

- Generic Relationships
- Instance Relationships
- Whole-Part Relationships

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