The National Union Catalog, Pre-1956 Imprints: A Cumulative Author List Representing Library of Congress Printed Cards and Titles Reported by Other American Libraries. Compiled and Edited with the Cooperation of the Library of Congress and the National Union Catalog Subcommittee of the Resources Committee of the Resources and Technical Services Division, American Library Association

The National Union Catalog, Pre-1956 Imprints, often referred to by scholars as the Mansell sequence, represents one of the most ambitious bibliographical undertakings of the twentieth century. Published between 1968 and 1981 by Mansell Information/Publishing Ltd. in London, this massive reference set consolidated millions of library catalog records into a single, comprehensive sequence. The project was conceived to provide a definitive record of the holdings of the Library of Congress and other major North American research libraries for books published prior to 1956, serving as a cornerstone for bibliographic control in an era before digital databases.

The design and editorial execution of the series were overseen by the American Library Association's Subcommittee of the Resources Committee, which worked in close collaboration with the Library of Congress. The editorial effort involved the painstaking integration of printed cards and reports from various institutions, a task that required significant standardization of complex bibliographic data. Upon its release, the set was widely praised by the library and academic communities for its unprecedented scope, ultimately functioning as a primary tool for interlibrary loans and scholarly research, despite the eventual transition of this data into modern digital catalogs.