National Union Catalog

The National Union Catalog (NUC) is a printed catalog of books catalogued by the Library of Congress and other American and Canadian libraries, issued serially beginning in the 1950s. It is not related to the National Union Catalog of Manuscript Collections (NUCMC). The National Union Catalog of Pre-1956 Imprints, a set of 754 volumes, largely superseded the older Library of Congress Catalog of printed books, and included printed works published before 1956 which are held by major American and Canadian libraries. It is sometimes referred to as the Mansell, after its publisher. This set is a massive bibliography compiled during the period from 1968 to 1981. It contains photocopies of printed catalog cards from major American and Canadian libraries, arranged alphabetically by author's last name, or by title for books that have no author, such as the Bible.

The NUC of Pre-1956 Imprints was an important resource for verifying bibliographic information and finding copies of books before the advent of large electronic bibliographic databases, such as WorldCat; the massive size and weight of the set make it less useful now. However, given that approximately 27% of the books listed in the NUC Pre-1956 Imprints were not in listed WorldCat as of 2005, it remains an extremely valuable tool for researchers.