What is a controlled vocabulary?
A controlled vocabulary is an organized set of terms (words and phrases) meant to facilitate description and retrieval of items in a collection. These terms can be in alphabetical or hierarchical order, and are often developed to describe a certain type of collection, such as the Art & Architecture Thesaurus or the Nomenclature for Museum Cataloging (although that does not mean that they can only be used for that type of collection). Controlled vocabularies can be used to describe the subject of an item, the form or genre of an item, or even provide information about its provenance.
Subject headings are words used for describing the subject of an item, in order to retrieve groups of items on a particular topic (i.e. books about Shakespeare, or art depicting Romeo and Juliet). Subject heading lists are usually arranged alphabetically rather than thematically or hierarchically; they provide the preferred terms for topics along with cross-references to alternative terms. The Library of Congress Subject Headings are probably the best-known example of this kind of vocabulary.
A thesaurus, in contrast, organizes its terms hierarchically, since its main emphasis is on the relationships between terms. Like a subject heading list, it provides preferred terms and cross-references, but also includes relationship indicators with each term. For instance, the Art & Architecture Thesaurus indicates that Bibliographies are a form of the concept Lists (which are in turn a Document Genre). A thesaurus represents a semantic network, not just a flat list of terms.
If you'd like to read about controlled vocabularies in more detail, the Getty Institute has produced a comprehensive and freely-available Introduction to Controlled Vocabularies.
Controlled vocabularies at the Folger
Folger catalogers use several controlled vocabularies (both subject heading lists and thesauri) to fit the needs of different materials. These controlled vocabularies are used in several ways:
Controlled vocabularies may appear in MARC fields 600, 610, 630, 650, 651, and 655. When they are from any vocabulary other than LCSH, the source vocabulary is indicated in a subfield ‡2 following the term (see the Subject heading term source code list and the Genre/form term source code list for more information). The Folger generally uses non-LCSH terms as genre/form headings only; they can be found in the 655 field.
The Library of Congress Subject Headings thesaurus is a general thesaurus, first developed by the Library of Congress in 1898, and actively maintained and updated since then. At the Folger, it is used to specify subjects for both open stacks and Vault materials.
600 10 Shakespeare, William, 1564-1616 ‡x Homes and haunts ‡v Pictorial works
650 0 Political corruption ‡z England ‡v Early works to 1800
In the two examples above, notice that there is a subfield ‡v present. LCSH is unusual in that it does not include only subject terms, but also genre terms and chronological and geographical divisions. The subfield ‡v indicates a genre/form term: "Pictorial works" indicates that images are a central part of an item, and "Early works to 1800" indicates that the item is a work printed or issued before 1800. Even though the items are not necessarily about pictures or early modern printing, they are examples of them. Genre subdivisions are gradually being phased out of practice as the Library of Congress develops more specialized vocabularies and library catalogs implement faceted browsing, but for now many are still actively used.
The LCSH is a broad, generalized vocabulary, and it can describe a variety of materials. However, it can be difficult to describe certain items fully using only LCSH, especially in the case of the Folger's specialized early modern collections. To this end, Folger catalogers turn to a selection of smaller but more specialized vocabularies: the RBMS controlled vocabularies, the Art & Architecture Thesaurus, and occasionally the Thesaurus for Graphic Materials (as well as a small handful of local terms).
The Rare Book and Manuscripts Section (part of the American Library Association) maintains six controlled vocabularies relating to the physical evidence, providence, and genre of rare materials. RBMS terms are used extensively by the Folger as genre and form terms to describe our Vault collections, and some of our open stacks items as well (such as to note the presence of an author's inscription). Terms from the Genre vocabulary are added to records as-is, but terms from the other RBMS vocabularies include the vocabulary name in following parentheses: i.e., "Prayer books" vs. "Prize books (Provenance)." The RBMS vocabularies account for seven abbreviations on the list of source codes for genre & form terms (printing and publishing terms get distinct abbreviations, despite being combined into a single thesaurus).
Composition errors (Printing). ‡2 rbpri
Printed waste (Binding). ‡2 rbbin
Volvelles. ‡2 rbgenr