The MARC (MAchine-Readable Cataloging) encoding system has been used to create electronic catalog records since the mid-1960s. The most current version for use in the United States is MARC 21, a harmonized version of USMARC and CAN/MARC, first published in 1999 and continually revised. Currently, the MARC encoding system holds the position of being the one used for bibliographic records in the vast majority of the world’s online catalogs, although this is poised for change.
MARC21 consists of five specific formats:
- Bibliographic format: for encoding bibliographic data in records that are surrogates for information resources
- Authority format: for encoding authority data collected in authority records created to help control the content of those surrogate record fields that are subject to authority control
- Holdings format: for encoding data elements in holdings records that show the holdings and location data for information resources described in surrogate records
- Community information format: for encoding data in records that contain information about events, programs, services, and the like
- Classification data format: for encoding data elements related to classification numbers, the captions associated with them, their hierarchies, and the subject headings with which they correlate
Each MARC format is made up of three structural elements: the record structure, the content designation, and the actual metadata content.
- Record structure refers to MARC's implementation of the Format for Information Exchange (ISO 2709) and Bibliographic Information Interchange (ANSI/NISO Z39.2), which address the technical aspects of the MARC standard.
- Content designation refers to semantics. It addresses the use of tags and subfield codes to specifically identify and label the metadata held in parts of MARC records. Content designation allows automated library systems to process, index, and manipulate the data found in the record as needed.
- Content refers to the metadata contained in the record, formatted according to content standards, controlled vocabularies, classification schemes, and the like.