When a genre frequently has the same physical characteristics associated with it, both aspects are not necessarily indexed.

Example: Most POSTCARDS (genre term) are PHOTOMECHANICAL PRINTS (physical characteristic term), and many CATALOGS (genre term) are PAMPHLETS (physical characteristic term). In such cases, the genre term alone may be sufficient.

Example: For a less common combination of genre and physical characteristic, such as albumen stereographs, both STEREOGRAPHS (genre) and ALBUMEN PRINTS (physical characteristic) are used.

The same principle applies to terms for the instrument of application. Since most GRAPHITE DRAWINGS are made with pencils, it is not necessary to index under PENCIL WORKS unless there is a particular reason to draw attention to that aspect.

Terms are related in the thesaurus (RT) when their definitions overlap and, sometimes, when the categories frequently occur together. It is generally not necessary to index under both terms; rather, the researcher should be encouraged to use the thesaurus to be guided from one term to the other.

Example: STEREOGRAPHS, which are predominantly CARD PHOTOGRAPHS, are linked through the RT notation and are not double indexed with both terms.