Section 6
BT, NT, and RT References

What Is the Relationship Between BT and NT?

Normally, BT and NT are "inverse" links. In other words, if X is a broader term to Y, then Y is a narrower term to X, and vice versa.

For example, if a thesaurus contains the entry

PENS
BT WRITING MATERIALS
you would expect it also to have the entry
WRITING MATERIALS
NT PENS

How Many BT References Can a Term Have?

A thesaurus is usually "polyhierarchical"; this means that a term can have more than one immediately broader term and more than one BT reference. For example,
SOCIAL PSYCHOLOGY
BT PSYCHOLOGY
BT SOCIOLOGY

Polyhierarchy avoids futile arguments about the "best" broader term to choose.

Some terms in a thesaurus have no broader terms and so no BT references. Such terms are usually fairly broad in meaning, at least within the subject area covered by the thesaurus. For example, in a sports thesaurus, "SPORTS" might have no broader terms.

When Should BT/NT References Be Omitted?

You should not indicate every hierarchical relation explicitly in your thesaurus. That could make the entries too long and difficult to read. Instead, omit those links that are implied by other links.

Suppose X is a broader term to Y, which in turn is a broader term to Z. Do not make BT/NT references between X and Z.

For example,

PLANT PRODUCTS
NT FRUIT
and
FRUIT
NT FRESH FRUIT
but not
PLANT PRODUCTS
NT FRESH FRUIT

When To Use an RT Reference

An RT reference is used for non-hierarchical semantic relations in a thesaurus. To decide whether there should be an RT reference between two preferred terms X and Y that do not have a hierarchical relation, you can use the following test:
Should an indexer or a searcher considering using X be reminded of the existence of Y?

What Is the Relationship between RT and RT?

Normally, RT is its own "inverse" link type. In other words, if X has an RT reference to Y, then Y should have an RT reference to X. For example, if a thesaurus contains the entry
PENS
RT CALLIGRAPHY
you would expect it also to have the entry
CALLIGRAPHY
RT PENS

Semantic Categories of RT References

In constructing your thesaurus, you may find it useful to list some categories of semantic relations that you think should be covered by RT references.

Here are some categories sometimes used, with examples.
Categories Examples
Time LEISURE READING
RT LEISURE TIME
Place FOREIGN LANGUAGES
RT LANGUAGE LABORATORIES
Product STILL CAMERAS
RT PHOTOGRAPHS
SHIPBUILDING
RT SHIPS
Cause VANDALISM
RT HOSTILITY
Agent COACHING
RT COACHES
Device PAINTING
RT PAINT BRUSHES
Application COMPUTERS
RT WORD PROCESSING
Part VEHICLES
RT WHEELS
Complement PARENTS
RT CHILDREN

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Section 5 Section 7 Table of Contents Glossary
Last updated January 25, 2008, by Tim Craven
Copyright © 1997 The University of Western Ontario