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A catalog (or catalogue — the former spelling is infrequently found in British writing but is fairly common in America), in general, is an organized register of some set of objects (for example, books, works of art, coins, car parts).
A library catalog is a register of all bibliographic items found in a library. A bibliographic item is a library material (for example, a book), a part of a library material (for example, a single novel in an anthology), or a group of library materials (for example, a trilogy), as far as it is relevant to the catalog.
A library catalog typically serves to answer the following questions:
Some large libraries possess almost all publications of some period, or some country, and their catalog also serves to answer the following questions:
In addition, a catalog may serve internal purposes of the library, for example as an inventory of everything that should be present.
Traditionally, there are the following types of catalog:
Producing formal catalogs is relatively easy, as the cataloger can follow a set of cataloging rules. However, a formal catalog cannot answer question 3 (which works about some subject does the library have?). A subject catalog just serves that goal, but it is much more difficult to produce, as the cataloguer has to get an accurate impression of the contents of the bibliographic item.
Cataloging rules have been defined to allow for consistent cataloging of various library materials across several persons of a cataloging team and across time. Users can use them to clarify how to find an entry and how to interpret the data in an entry. Cataloging rules prescribe
The larger a collection, the more elaborate cataloging rules are needed. Users cannot and do not want to examine hundreds of catalog entries or even dozens of library items to find the one item they need.
Currently, most cataloging rules are similar to, or even based on, the International Standard Bibliographic Description (ISBD), a set of rules produced by the International Federation of Library Associations (IFLA) to describe a wide range of library materials. These rules organize the bibliographic description of an item in the following areas: title and statement of responsibility (author or editor), edition, material-dependent information (for example, the scale of a map), publication and distribution, physical description (for example, number of pages), series, note, and standard number (ISBN).
Library items that are written in a foreign script are, in some cases, transliterated to the script of the catalog.
In a title catalog, one can distinguish two sort orders:
The grammatic sort order has the advantage that often, the most important word of the title is also a good keyword (question 3), and it is the word most users remember first when their memory is incomplete. However, it has the disadvantage that many elaborate grammatic rules are needed, so that only expert users may be able to search the catalog without help from a librarian.
In some catalogs, person's names are standardized, i. e., the name of the person is always (catalogued and) sorted in a standard form, even if it appears differently in the library material. This standardization is achieved by a process called authority control . An advantage of the authority control is that it is easier to answer question 2 (which works of some author does the library have?). On the other hand, it may be more difficult to answer question 1 (does the library have some specific material?) if the material spells the author in a peculiar variant. For the cataloguer, it may incur (too) much work to check whether Smith, J. is Smith, John or Smith, Jack.
For some works, even the title can be standardized. For example, translations and reeditions are sometimes sorted under their original title. In many catalogs, parts of the Bible are sorted under the standard name of the book(s) they contain.
Many detail questions about alphabetic sorting of entries arise. Some examples:
In a subject catalog, one has to decide on which classification to use.