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MODS: Uses and Features

The Library of Congress' Network Development and MARC Standards Office, with interested experts, is developing a schema for a bibliographic element set that may be used for a variety of purposes, and particularly for library applications. As an XML schema it is intended to be able to carry selected data from existing MARC 21 records as well as to enable the creation of original resource description records. It includes a subset of MARC fields and uses language-based tags rather than numeric ones, in some cases regrouping elements from the MARC 21 bibliographic format. This schema is currently in draft status and is being referred to as the "Metadata Object Description Schema (MODS)". MODS is expressed using the XML schema language of the World Wide Web Consortium. The standard is maintained by the Network Development and MARC Standards Office of the Library of Congress with input from users.

1. Uses

MODS could potentially be used as follows:

  • as an SRU specified format
  • as an extension schema to METS (Metadata Encoding and Transmission Standard)
  • to represent metadata for harvesting
  • for original resource description in XML syntax
  • for representing a simplified MARC record in XML
  • for metadata in XML that may be packaged with an electronic resource

2. Advantages of MODS

MODS is intended to complement other metadata formats. For some applications, particularly those that have used MARC records, there will be advantages over other metadata schemes. Some advantages are:

  • The element set is richer than Dublin Core
  • The element set is more compatible with library data than ONIX
  • The schema is more end user oriented than the full MARCXML schema
  • The element set is simpler than the full MARC format

3. Features of MODS

  • The elements generally inherit the semantics of MARC
  • Some data has been repackaged; in some cases what is in several data elements in MARC may be brought together into one in MODS
  • MODS does not assume the use of any specific cataloging code
  • Several elements have an optional ID attribute to facilitate linking at the element level.

4. Limitations of MODS

MODS includes a subset of data from the MARC 21 Format for Bibliographic Data. As an element set that allows for the representation of data already in MARC-based systems, it is intended to allow for the conversion of core fields while some specific data may be dropped. As an element set for original resource description, it allows for a simple record to be created in some cases using more general tags than those available in the MARC record.

However, the schema does not target round-tripability with MARC 21. In other words, an original MARC 21 record converted to MODS may not convert back to MARC 21 in its entirety without some loss of specificity in tagging or loss of data. In some cases if reconverted into MARC 21, the data may not be placed in exactly the same field that it started in because a MARC field may have been mapped to a more general one in MODS. However the data itself will not be lost, only the detailed identification of the type of element it represents. In other cases the element in MARC may not have an equivalent element in MODS and then the specific data could be lost when converting to MODS.

MODS does not include business rules for populating the elements. The fields listed under "xsd:documentation" are intended as comments, to refer the user to where the semantics for the elements listed may be found. These MARC 21 elements listed are not intended to provide a crosswalk between MODS and MARC; additional instructions would need to be provided for conversion details.

5. Order of Elements

Note that the order of elements in the MODS schema does not assume display order. A stylesheet would be used to control display order of MODS records.

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