Access to graphic materials in libraries and archival collections frequently has been limited to retrieval by subject content and names of creators. Although catalog records often include information on genre and physical characteristics, researchers have not always had ready access to it. A student of lithography, for example, may be compelled to consult reference books for names of printmakers likely to have produced lithographs, then to search a library's catalog of prints for artists' names, and, finally, examine each catalog record to find those for lithographs. A scholar investigating the cultural impact of photographically illustrated books may be forced to rely on a few published bibliographies, in-house example files, staff memory, and chance discoveries to locate examples of such works. As graphic collections grow and catalog records accumulate, it is clear that additional access points greatly facilitate research related to functional categories, production contexts, and artifactual aspects of graphic materials.
A single list of standard terms from which catalogers and researchers can choose indexing and retrieval vocabulary was needed, along with widely accepted provisions for applying the terms as access points. The first edition of Descriptive Terms for Graphic Materials: Genre and Physical Characteristic Headings (GMGPC) appeared in 1986. Before then, the lack of such a list and cataloging guidelines was a problem because of the great variety of media and pictorial types and because of the broad range of users of graphic materials, whose knowledge and experience vary. While an extensive and often informal vocabulary is employed in the descriptive portion of the catalog record, indexing terms should be controlled. Reconciling variant terms by designating a preferred indexing term not only simplifies the cataloger's task but also makes retrieval more efficient. For example, the standard term "dry plate negatives" collocates "silver gelatin glass negatives," "glass plates," and "dry plates." Furthermore, the terms for indexing graphic materials can be used most effectively when presented within the structure of a thesaurus designed to establish relationships and guide users.
While TGM II can be used in a variety of cataloging systems, it was created primarily in response to the needs of institutions using the communications format called MAchine-Readable Cataloging (MARC) for their automated catalog records. In 1979, the Independent Research Libraries Association (IRLA) recommended in "Proposals for Establishing Standards for the Cataloguing of Rare Books and Specialized Research Materials in Machine-Readable Form" the addition of two new fields to the MARC format for terms indicating genre and publishing/physical aspects. In 1980, the field 655 was authorized for genre headings and in 1984 field 755 was authorized for physical characteristic headings. In 1995, the need and practicability for keeping these two fields of data separate was determined to be insufficient, and the 755 field was made obsolete. The 655 "Form-Genre" field is now defined for all types of material in the USMARC Format for Bibliographic Data.
IRLA asked the Standards Committee of the Rare Books and Manuscripts Section of the Association of College and Research Libraries to develop thesauri appropriate for rare books and special collections. The lack of vocabulary to index book illustrations led the Committee to encourage staff of the Library of Congress Prints and Photographs Division to expand and integrate several of their genre and physical description lists into a thesaurus constructed according to guidelines set out by the American National Standards Institute. The result was Descriptive Terms for Graphic Materials: Genre and Physical Characteristic Headings with 513 authorized terms and 290 cross references. When the second edition, TGM II, closed for publication in June 1994, it had 600 authorized terms and 448 cross references, a 15% increase in postable terms. In July 2004, there are 660 main terms.
The terms added or changed since 1994 are listed online at https://www.loc.gov/rr/print/tgm2/newterms.html.