Some pictorial materials are important as much for their artifactual value as for their subject content. The distinction between topical terms and genre or physical characteristic terms is critical in describing such materials, since researchers often know with great certainty whether they wish to see examples of formats and physical types or images in which formats or physical types constitute the subject. When a pictorial item is an example of a format or physical type, this may be indexed by selecting the appropriate term from Thesaurus for Graphic Materials II: Genre and Physical Characteristic Terms (TGM II) (entered in MARC field 655). When an image depicts recognizable types of graphic materials, a term from TGM II may be used as a topical heading (entered in MARC field 650).
Example: A stereograph of a family viewing stereographs is indexed with: (650) Stereographs (for the subject of the image); (655) Stereographs (for the physical characteristic of the item).
Example: A self-portrait of an artist making a self-portrait is indexed with: (650) Self-portraits (for the subject of the image); (655) Self-portraits (for the format of the item).