Whether automated or in traditional card format, the library's catalog is a bibliography and the portal to the library's collection. It provides several access points: for the automated catalog or online public access catalog (OPAC), the traditional author and related responsible persons, title of individual works or series, standardized subject headings, plus keyword access to all parts of the record; and, for the card catalog, the author, title, standardized subject headings, and series.
Any catalog - including your own - can be used not only to identify whether your library, or another library, owns a particular book, but as a resource for answering reference questions. If you are searching for books on a particular topic, a keyword search (using an OPAC) or a search of subject headings will produce a list of titles. To search for additional or related titles, use the subject headings assigned to the books you have already identified.
A catalog can list the titles owned by one library or a number of libraries. A catalog which contains the holdings of many libraries is known as a union catalog - that is, in addition to including standard catalog records, it shows which libraries own each item. WorldCat is the primary example of a contemporary union catalog.
OCLC is the world's largest bibliographic utility, containing the merged catalogs of thousands of libraries worldwide; WorldCat is its end-user catalog. There are nearly 50 million records in WorldCat. These records contain traditional catalog data, and may also include information about item contents.
Think of WorldCat as a library catalog; use it to identify items that are not in your library and to verify information about books and other items. Like other catalogs, you can search it by multiple access points such as keywords in the author, title, and subject fields. WorldCat includes records for Web sites and other electronic resources as well as books. Many recent records also include a short annotation. WorldCat also shows the names of libraries owning each item. If Idaho libraries own an item, their names are always displayed at the top of the list of holdings. Entries are not complete MARC records, but they all show subject headings and most give both Dewey and Library of Congress classification numbers.
There are a number of United States union catalogs of varying titles that were an outgrowth of the original printed Library of Congress Catalog. These are collectively known as the National Union Catalog (NUC). These catalogs are arranged by main entry (usually author), and the pages contain reproductions of the catalog cards. Each multi-volume NUC set takes up a number of shelves, and all together they occupy significant amounts of shelf space. Although seldom used today, the NUC played a defining role in resource sharing and shared cataloging before the advent of bibliographic utilities such as OCLC.
While most bibliographic information can be found using WorldCat, it's important for library staff to be aware of the National Union Catalog. Remember, too, that you can search the Library of Congress Online Catalog (http://catalog.loc.gov).
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