Machine-Readable Cataloging (MARC) has been the standard in library automation since the late 1960s. It forms the backbone of today's automated library systems, networks, and bibliographical utilities around the world. The MARC format is the data communication protocol for "translating" the text of a catalog record for use in an online catalog. The MARC products developed at the Library of Congress set the standard for MARC cataloging documentation. The Library of Congress, the British Library, and the National Library of Canada harmonized the USMARC, UKMARC, and CAN/MARC formats, and joined their MARC documentation to form MARC 21. All USMARC products have become MARC 21 products.
The Library of Congress Online Catalog is a database of records representing the vast collection of materials held by the Library. In addition to these records, the Online Catalog provides cross-references, notes, and circulation status, as well as information about Library materials still in the acquisitions stage. It is currently available in two forms, the original and a responsive design. Functionality has been built in to allow libraries to save records or export them for use using Z39.50/SRU; for full details read "About the LC Online Catalog."