LCThe Thesaurus for Graphic Materials (TGM) is a controlled vocabulary developed and maintained by the Library of Congress Prints & Photographs Division for indexing visual materials by subject and by genre/format. It is the standard indexing tool for pictorial collections at the Library of Congress and is widely used by archives, libraries, and museums that manage photographs, prints, posters, drawings, and other graphic materials.
Background
The TGM has its origins in two separate thesauri developed by the Library of Congress. TGM I (Thesaurus for Graphic Materials I: Subject Terms) was first published in 1995, providing a controlled vocabulary for subject access to pictorial materials. TGM II (Thesaurus for Graphic Materials II: Genre and Physical Characteristic Terms) addressed the need to describe types of visual materials -- the distinction between a daguerreotype and a gelatin silver print, for example, or between an editorial cartoon and a portrait.
In 2007, the two vocabularies were merged into a single unified thesaurus and migrated to MultiTes software, simplifying maintenance and use. The merged TGM continues to be updated with new terms as needed, with proposals accepted from the cataloging community.
Purpose & Scope
The TGM serves as a bridge between researchers seeking visual materials on particular topics and the cataloging records that describe those materials. It provides standardized terms for two distinct access dimensions:
Subject terms (over 7,000) describe what a visual item depicts or is about -- people, places, events, activities, objects, and concepts. Examples include terms such as "Aerial photographs," "Coal mines," "Political campaigns," and "Street vendors."
Genre/format terms (approximately 650) describe the type, technique, or physical characteristics of a visual item. Examples include "Albumen prints," "Lithographs," "Stereographs," and "Woodcuts."
Both types of terms include scope notes explaining usage, cross-references to broader, narrower, and related terms, and references to equivalent or nearly equivalent terms in the Library of Congress Subject Headings (LCSH) where applicable.
Structure & Organization
The TGM follows established thesaurus standards with hierarchical (broader/narrower) and associative (related term) relationships. Terms include:
| Field | Description |
|---|---|
| Preferred Term | The authorized form of the heading |
| Use For (UF) | Variant terms and synonyms |
| Broader Term (BT) | Parent term in the hierarchy |
| Narrower Term (NT) | More specific child terms |
| Related Term (RT) | Associatively linked terms |
| Scope Note (SN) | Usage guidance and definitions |
The vocabulary uses subdivisions to extend subject terms geographically, chronologically, and topically, enabling compound headings such as "Railroads--California--1880-1900."
Governance & Maintenance
The TGM is maintained by editors in the Library of Congress Prints & Photographs Division. The editorial team accepts new term proposals through a published form, considering factors such as warrant from actual cataloging needs, specificity, and relationship to existing terms. While the thesaurus does not follow a fixed annual release cycle, it is updated on an ongoing basis. The thesaurus is freely available for unrestricted online access and can be downloaded in its entirety.
Notable Implementations
The TGM is the primary subject and genre vocabulary used in the Library of Congress Prints & Photographs Online Catalog (PPOC), which provides access to over 1.2 million images. Many other institutions that follow LC cataloging practices for visual materials use TGM, including the National Archives, the Smithsonian, university special collections, and state historical societies. TGM terms are also used as subject access points in MARC records for graphic materials.
Related Standards
The TGM is closely related to the Library of Congress Subject Headings (LCSH), with many TGM subject terms having corresponding LCSH equivalents. It is also related to the Categories for the Description of Works of Art (CDWA) vocabulary framework and to the Art & Architecture Thesaurus (AAT) from the Getty Research Institute, which provides a more granular vocabulary for art and architectural terminology. For moving image materials, the Library of Congress maintains separate genre terms through the Moving Image Genre-Form Guide.