MARC Format Reference

MARC 21 (Machine-Readable Cataloging) is the international standard format for structuring bibliographic data in library catalogs. Developed by the Library of Congress, it enables libraries worldwide to create, share, and exchange catalog records through integrated library systems (ILS). MARC 21 encodes information like author, title, subject, publisher, and physical description in a structured format that computers can process while maintaining the richness of traditional cataloging. It is based on ISO 2709 for record structure and is used by virtually all library automation systems.

The Leader is a fixed 24-byte field at the beginning of every MARC record. Key positions include: 00-04 (record length), 05 (record status: n=new, c=corrected, d=deleted), 06 (record type: a=language material, g=projected medium, m=computer file), 07 (bibliographic level: m=monograph, s=serial), and 17 (encoding level: blank=full, 3=abbreviated). The Leader is not tagged like other fields -- it is always present and always exactly 24 characters.

Both fields record publication information (place, publisher, date), but 264 was introduced with RDA (Resource Description and Access) cataloging rules to provide more precise function designation. Field 264 uses its second indicator to specify the role: 1=production, 2=publication, 3=distribution, 4=manufacture. Field 260 was used under AACR2 rules and does not distinguish these functions. In current practice, 264 is preferred for new records following RDA, while 260 remains in legacy records.

Each MARC data field has two indicator positions, each a single digit (0-9) or blank (represented as backslash). The meaning of indicators varies by field. For example, in field 245 (title), the first indicator controls whether an added entry is generated (0=no, 1=yes), and the second indicator specifies the number of non-filing characters to skip for sorting (0-9, accounting for articles like "The" or "A"). In field 100 (personal name), the first indicator specifies the name entry order (0=forename, 1=surname, 3=family name).

Subfields are subdivisions within a MARC field, each identified by a delimiter ($ or double-dagger symbol) followed by a lowercase letter or digit. For example, in field 245, $a is the main title, $b is the subtitle, and $c is the statement of responsibility. In field 264, $a is place of publication, $b is publisher name, and $c is publication date. Common subfields across many fields include $6 (linkage) and $8 (field link and sequence number). Some subfields are repeatable within a field while others are not.

Field 490 is a series statement as it appears on the item (display form), while field 830 is the series added entry in its authority-controlled form (access point). When 490 first indicator is 1, it signals that a corresponding 8XX added entry exists. For example, 490 might contain the exact wording from the title page, while 830 contains the standardized form used for catalog searching and collocation. This separation allows the display to match the item while the access point follows authority file conventions.

Subject headings use the 6XX field range. Field 650 is for topical subjects, with the second indicator specifying the thesaurus source: 0=LCSH (Library of Congress Subject Headings), 2=MeSH (Medical Subject Headings), 7=source specified in $2 (e.g., $2 nlsh for Korean National Library Subject Headings). Subfield $a contains the main heading, $x is for general subdivisions, $z for geographic subdivisions, and $y for chronological subdivisions. Field 600 is used when a person is the subject of the work.

MARC records are primarily distributed in two formats: ISO 2709 binary (.mrc files), which is the traditional format with Leader, Directory, and data fields using ASCII control characters (field separator ASCII 30, record terminator ASCII 29), and MARCXML (.xml files), which is an XML representation of the same data. The Directory in ISO 2709 format consists of 12-byte entries per field containing the tag (3 bytes), field length (4 bytes), and starting position (5 bytes). Most modern ILS systems can import and export both formats.