TGM I terms are suited to post-coordinate indexing: terms from the thesaurus may be listed separately to cover all aspects of the subjects represented in the image without indicating any relationships that may exist among the subjects; searchers may then combine terms at the searching stage. For example, an image showing women involved in sports activities might be assigned two separate headings: Women and Sports. TGM I terms may also be used as elements in subject heading "strings" in order to bring out relationships among topics, as in the subject heading string: Women-- Sports. A number of factors must be weighed in determining whether to rely on the single term or the subject heading string approach, including the data entry, retrieval, and display features of the system in which the terms will be used and the kinds of knowledge and skills that will be required of indexers and users of the system.
Subject heading strings that link aspects of the subject covered (e.g., names, topics, time periods, places) and succinctly display these relationships have long been used in manual indexing systems. Even in an age when many institutions use automated systems with Boolean and keyword searching capabilities that enable searchers to specify desired combinations of subject characteristics at the searching stage, subject heading strings can still prove of value. Explicitly linking together aspects of a subject may help searchers avoid, or at least more quickly identify and weed out, "false drops"--retrieval results where the relation between two or more subjects specified by the searcher is not what the searcher intended. The researcher interested in images of women engaging in sports may find a number of records that include the subjects Women and Sports, including images of women watching sports and images of women picketing sports events; the subject heading string Women--Sports, however, enables the researcher to identify quickly those images that show women doing sports. Linkage of this sort can be particularly useful when a single catalog record covers multiple subjects taking place in multiple locations at multiple times, as is sometimes the case in records for groups of images.
Searchers should not have to construct and enter long strings of terms and subdivisions just to see what might be available on their topic. But such strings can be useful in orienting the searcher when a large result is returned in response to a keyword search. Although there is some evidence that multiple screens of subdivided headings confuse users of online catalogs, it is also true that well-designed, easily browsed displays of subject heading strings can help users to (1) develop a conceptual framework of how information in the catalog is organized and (2) winnow large sets of results down to the material most appropriate to the searcher's need. For instance, a display such as:
Women--Political activity Women--Social life Women--Spiritual life Women--Sports
might be doubly useful to the researcher interested in depictions of women, making it possible to home in on the particular aspect of the topic most of interest and, at the same time, suggesting possible related aspects.
Significant as the virtues of precoordinated terms may be for retrieval and display, some institutions may find that their system is not designed to take advantage of the benefits. Others may find that the benefits are outweighed by the effort required to maintain consistency in the elements and sequence of elements included in strings or by the extra data entry involved in repeating geographic and chronological subdivisions under multiple topical headings in the same record. Outlined below are some of the subdivision practices employed by the Prints and Photographs Division. Use of nationality, geographic, chronological, and topical subdivisions, however, is optional.