Developing and maintaining a large subject thesaurus is a major effort; it is one that the Library of Congress Prints and Photographs Division (P&P) undertakes out of an ongoing need not fulfilled by other tools. Understanding the origins and evolution of P&P's thesaurus work, the primary sources and characteristics of terminology included in TGM I, as well as TGM I's relationship to other thesauri, may help in evaluating its usefulness for particular applications.
TGM I is built from a base of vocabulary that has been used to provide subject access to P&P's collections in the course of over 50 years of cataloging and indexing. The terms found in TGM I thus reflect the diversity of the subject matter found in materials held by P&P, which range from documentary photographs to architectural drawings to editorial cartoons to fine prints. Since terms are added to TGM I only as the topic is encountered in the course of cataloging and indexing, not all possible topics are represented in the thesaurus. Likewise, TGM I vocabulary is quite specific in some topic areas and remains very general in others because of the varying nature of P&P's indexing activities. Depending upon the situation, the indexing may cover anything from a single item to an entire collection.
The first comprehensive, systematic listing of P&P's subject terminology was compiled by Elisabeth Betz Parker and was issued in 1980 as a preliminary list entitled Subject Headings Used in the Library of Congress Prints and Photographs Division (SHP&P). The first edition of TGM I (published in 1987 and referred to then by the acronym LCTGM) incorporated the topical terms that were found in SHP&P. But TGM I excluded the proper names (e.g., Halloween, Ku Klux Klan, Niagara Falls, Virgin Mary, War of 1812) that had been included in the earlier list. In conformity with the ANSI standard, TGM I also eliminated SHP&P's inverted and compound term structures and shifted from constructing strings composed of broader and narrower terms (e.g., Animals--Camels, Animals--Cats) to showing the relationship among broader and narrower terms (as well as other term relationships) using ANSI standard thesaurus notation.
Another development was in the handling of terms for visual genre and physical characteristics and techniques (e.g., Daguerreotypes, Memorial prints, Posters), many of which had been included in the preliminary list. Because of the distinctive nature of this vocabulary, it was developed before TGM I within the framework of a separate thesaurus, Descriptive Terms for Graphic Materials: Genre and Physical Characteristics Headings (GMGPC). The first edition of TGM I included all of the terms and cross-references that appeared in GMGPC for ease of reference and for use in cases where types and formats of graphic materials constitute the subjects of images (see Section II.E). That practice is continued in this edition; postable and non-postable terms found in Thesaurus for Graphic Materials II: Genre and Physical Characteristic Terms (the second edition of the Descriptive Terms thesaurus) as of October 1994 are included in TGM I. These terms are identifiable by the statement "TGM II term" that appears in the cataloger's note (CN).
P&P's thesauri not only complement each other, they also complement other published thesauri and subject heading lists. While many of the topics covered by TGM I are no different from those that occur in other media, some of the more specific, visual concepts, such as Document signings, Dormers, Hammer & sickle, Moonlight, Shaking hands, and Ship of state, occur much more frequently as subjects of images than as subjects of books or other formats. Such terms are, therefore, not found in thesauri geared to indexing non-pictorial materials. Conversely, thesauri or subject heading lists designed for indexing other media, especially textual materials, include many topics that cannot be expressed visually. TGM I attempts to aid indexers and users of visual materials by constructing a cohesive network of terms representing subjects commonly expressed in still images.
Many institutions provide access to materials in a variety of media, with indexing supplied from diverse sources and, therefore, must maintain databases which include terminology from multiple subject thesauri. In order that terminology might be as compatible as possible in such an environment, TGM I incorporates terms from other standard thesauri whenever possible.
The encyclopedic Library of Congress Subject Headings (LCSH), which (as of the end of 1993) includes 206,300 postable terms devised primarily in the course of cataloging textual, book-length materials, is the major thesaurus used by the Library of Congress and numerous other institutions. As such, it is the most frequent source of terminology for TGM I. LCSH terms are used without alteration whenever possible, but term syntax or spelling is at times changed in order to conform to the ANSI standard. As necessary, cross references are made from LCSH terms to corresponding TGM I terms.
It should be noted that TGM I does not utilize two techniques authorized in LCSH for indexing publications concerned with or consisting chiefly of pictures. The LCSH free-floating form subdivision "Pictorial works" would be applicable to all pictorial materials and is far too broad to be a meaningful form heading for original and historical graphics. In addition, the technique of establishing a phrase heading "[topic] in art" (e.g., Cathedrals in art) is used in LCSH to designate textual materials which discuss or illustrate a specific subject as an artistic theme. Applying such a phrase to original materials would imply a value judgment about the particular work as "art" and would blur the distinction between objective subject analysis and iconographical analysis. For collections consisting largely or entirely of pictorial works, both of these techniques lose their meaning.
Lack of textual documentation frequently makes it difficult for a picture indexer to determine which aspect of a multifaceted subject best applies to the image in hand. In a thesaurus as extensive as LCSH, for example, one finds both scientific and popular names for plants and animals, as well as a variety of terms expressing closely related elements of a concept or phenomenon (e.g., Arbitration, Industrial; Collective bargaining; Industrial relations; Labor contracts; Labor disputes; Mediation & conciliation, Industrial). In some cases, elements occur so regularly together when represented in visual form that they need not be separately indexed. In order to guide picture indexers in their choice of terms and thereby improve the consistency of their indexing, TGM I selects from the rich vocabulary of LCSH, limiting the choice among overlapping terms (i.e., there are fewer to choose from) and attempting to make their specific application clear by means of notes and relationships.
The Legislative Indexing Vocabulary (LIV), developed for the Congressional Research Service at the Library of Congress, focuses on contemporary political and social issues. LIV is consulted in compiling TGM I because the types of terms LIV includes have been found to be useful for indexing pictorial materials such as posters and cartoons that relate to social and political issues.
Another valuable source of terminology is the Art and Architecture Thesaurus (AAT). Since the AAT is often compared to TGM I, it may be useful to clarify here differences in the scope, purpose, and method of compilation of the two thesauri. The AAT provides terminology "for art and architecture of the Western world from antiquity to the present." It is intended for use in indexing objects as well as textual and pictorial materials, incorporating highly specific concepts as well as the more general categories in which those concepts belong. The AAT includes an array of terms useful for indexing pictures of the built environment, furnishings and equipment, and manifestations of visual and verbal communication, as well as "supporting terminology" for the physical attributes, persons, and concepts that relate to the creation and appreciation of art and architecture. Lying outside the AAT's scope, however, are the broader range of people, events, and activities which are equally important aspects of general picture collections. The AAT is regularly consulted in adding art and architecture-related terms to TGM I and has been influential in thinking through relationships among TGM I terms.
The AAT is constructed independently of the cataloging operations of any single institution. AAT vocabulary has been developed to a considerable level of specificity in constructing the logical arrays of terms--the hierarchies--that compose it. In contrast, the terms included in TGM I are there, for the most part, because they have been needed in the course of cataloging the collections of the Prints and Photographs Division or a contributing institution. Occasionally a "gathering term" is added to TGM I when it is deemed useful for bringing out relationships among a cluster of narrower terms, but not all of the logically related terms or levels of hierarchy are incorporated unless they are needed for cataloging. This operating method partially accounts for the fact that the hierarchies that appear in TGM I are neither so "deep" nor so elaborate as those that have been developed for the AAT. Moreover, as the compilers of the AAT have pointed out, terms for abstract concepts are more difficult to classify than those for concrete objects. When no place for such terms can be found in an existing TGM I hierarchy, they are left as "orphans" (i.e., with no broader term designation) and an effort is made to lead the user to associated concepts through related term (RT) relationships.
In addition to the indexing tools mentioned above, standard tools which aid in term selection and definition include Webster's Third New International Dictionary, Webster's New Collegiate Dictionary, and the Encyclopedia Americana. The expertise of specialists in the Prints and Photographs Division, other divisions of the Library of Congress, and other institutions has also been tapped to identify commonly accepted terminology, define terms, and discern relationships among terms. In all cases, TGM I strives to avoid vocabulary which would imply subjective assumptions on the part of the indexer.