MARC (MAchine-Readable Cataloging) is a data format that emerged from a Library of Congress initiative to enable computers to exchange, use, and interpret bibliographic information. It defines the elements in most library catalogs. A MARC record uses tags and indicators to provide a guide for computers to interpret bibliographic data like author, title, subject headings. Fields can be fixed, containing codes for attributes, or variable, containing descriptive data. Examples show how tags like 245 for title and 100 for author main entry are structured in a MARC record.