MARC 21 is the product of the integration of USMARC, UKMARC and CANMARC (Canadian MARC). It is the most extensively used MARC format in the world and a de facto standard. It has been designed to be both a production format and an exchange format. There are five MARC 21 formats: 

  • Bibliographic
  • Authorities
  • Holdings
  • Classification
  • Community information

The current maintenance agency for MARC 21 is the Library of Congress which offers support documents on the formats with several translations including: Understanding MARC Bibliographic, Library of Congress and Understanding MARC Authority Records. The French translations of the MARC 21 formats are maintained by Library and Archives Canada.