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MARC (MAchine-Readable Cataloging) standards are digital formats for the description of bibliographic items developed by the US Library of Congress during the 1960s to facilitate the creation and dissemination of cataloging between libraries. By 1971, MARC formats had become the national standard for dissemination of bibliographic data and the international standard by 1973. There are several versions of MARC in use around the world, the most predominant being MARC 21, created in 1999 as a result of the harmonization of U.S. and Canadian MARC formats, and UNIMARC, widely used in Europe. The MARC 21 family of standards now includes formats for authority records, holdings records, classification schedules, and community information, in addition to the format for bibliographic records.
Record structure and field designations 
The MARC standards define three aspects of a MARC record: the record structure, the field designations within each record, and the actual content of the record itself.
Record structure 
MARC records are typically stored and transmitted as binary files, usually with several MARC records concatenated together into a single file. MARC uses the ISO 2709 standard to define the structure of each record. This includes a marker to indicate where each record begins and ends, as well as a set of characters at the beginning of each record that provide a directory for locating the fields and subfields within the record.
In 2002, the Library of Congress developed the MARCXML schema as an alternative record structure, allowing MARC records to be represented in XML. Libraries typically expose their records as MARCXML via a web service, often following the SRU or OAI-PMH standards.
Field designations 
Each field in a MARC records provides information about the item the record is describing. Since it was first developed at a time when computing power was low, and space precious, MARC uses a simple three-digit numeric code (from 001-999) to identify each field in the record. The bibliographic standard, for example, defines 100 as the primary author of a work, 245 as the title, 260 is used for publisher information, and so on.
Fields above 008 are further divided into subfields using a single letter or number designation. The 260, for example, is further divided into subfield 'a' for the place of publication, 'b' for the name of the publisher, and 'c' for the date of publication.
MARC is a metadata transmission standard, not a content standard. Other than a handful of fixed fields defined by the MARC standards themselves, the actual content a cataloger will place in each MARC field is usually governed and defined by standards outside of MARC. The Anglo-American Cataloguing Rules, for example, define how the physical characteristics of books and other item should be cataloged. The Library of Congress Subject Headings (LCSH) provides a list of authorized subject terms to describe the main content of the item. Other cataloging rules, subject thesauri, and classification schedules can also be used.
MARC formats 
|Authority records||provide information about individual names, subjects, and uniform titles. An authority record establishes an authorized form of each heading, with references as appropriate from other forms of the heading.|
|Bibliographic records||describe the intellectual and physical characteristics of bibliographic resources (books, sound recordings, video recordings, and so forth).|
|Classification records||MARC records containing classification data. For example, the Library of Congress Classification has been encoded using the MARC 21 Classification format.|
|Community Information records||MARC records describing a service providing agency. For example, the local homeless shelter or tax assistance provider.|
|Holdings records||provide copy-specific information on a library resource (call number, shelf location, volumes held, and so forth).|
MARC 21 
MARC 21 is a result of the combination of the United States and Canadian MARC formats (USMARC and CAN/MARC). MARC21 is based on the ANSI standard Z39.2, which allows users of different software products to communicate with each other and to exchange data. MARC 21 was designed to redefine the original MARC record format for the 21st century and to make it more accessible to the international community. MARC 21 has formats for the following five types of data: Bibliographic Format, Authority Format, Holdings Format, Community Format, and Classification Data Format. Currently MARC 21 has been implemented successfully by The British Library, the European Institutions and the major library institutions in the United States, and Canada.
MARC 21 allows the use of two character sets, either MARC-8 or Unicode encoded as UTF-8. MARC-8 is based on ISO 2022 and allows the use of Hebrew, Cyrillic, Arabic, Greek, and East Asian scripts. MARC 21 in UTF-8 format allows all the languages supported by Unicode.
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MARCXML is an XML schema based on the common MARC21 standards. MARCXML was developed by the Library of Congress and adopted by it and others as a means of easy sharing of, and networked access to, bibliographic information. Being easy to parse by various systems allows it to be used as an aggregation format, as it is in software packages such as MetaLib, though that package merges it into a wider DTD specification.
The MARCXML primary design goals included:
- Simplicity of the schema
- Flexibility and extensibility
- Lossless and reversible conversion from MARC
- Data presentation through XML stylesheets
- MARC records updates and data conversions through XML transformations
- Existence of validation tools
The future of the MARC formats is a matter of some debate among libraries. On the one hand, the storage formats are quite complex and are based on outdated technology. On the other, there is no alternative bibliographic format with an equivalent degree of granularity. The billions of MARC records in tens of thousands of individual libraries (including over 50,000,000 belonging to the OCLC consortium alone) create inertia. The Library of Congress has launched a Bibliographic Framework Transition Initiative (BibFrame).
See also 
- Henriette Avram
- Bibliographic control
- International Standard Bibliographic Description (ISBD)
- ISO 2709
- Metadata and metadata standards
- Taylor, Organization of Information, p77
- Character sets and encoding options, Part 2: MARC-8 Encoding Environment
- "Bibliographic Framework Transition Initiative". Library of Congress. Retrieved 2 February 2013.
- "Bibliographic Framework Transition Initiative". Library of Congress. Retrieved 3 December 2012.
Reitz, J. M.(2004)Online Dictionary for Library and Information Science. Available at: http://www.abc-clio.com/ODLIS/odlis_m.aspx
Further reading 
- MARBI (1996). MARC 21 Formats. Library of Congress.
- Taylor, Arlene G. (2004). The Organization of Information. Libraries Unlimited.
- Coyle, Karen (2011-07-25). "MARC21 as Data: A Start". The Code4Lib Journal (14).
- Tennant, Roy (2002-10-15). "MARC must die". Library Journal.
- Understanding MARC Bibliographic Machine Readable Cataloging, a good introduction
- MARC authority records
- "MARC 21 home page". Library of Congress.
- MARC frequently asked questions
- List of MARC country codes
- Network Development and MARC Standards Office
- MARC 21 Character Sets
- "Tools For MARC Records". Library of Congress.
- Kevin J. Comerford (12 September 1996). "Notes on MARC Format". bit.listserv.museum-l. Web link.
- Amazon to MARC Converter
- MAB information, Deutsche Nationalbibliothek
- MARC Records with Books From India, Pakistan, Nepal, Bhutan
- NISO/ANSI Z39.2
- Converting MARCBN into MARC21
- Library of Congress: MARCXML
- "Library of Congress Announces Standard MARCXML Schema"
- Interpreting MARC: Where’s the Bibliographic Data? by Jason Thomale Code4Lib Journal Issue 11, 2010-09-21 ISSN:1940-5758