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MODS User Guidelines (ver. 3) Home » Introduction and Implementation

Introduction and Implementation

MODS is an XML schema for a bibliographic element set that may be used for a variety of purposes, and particularly for library applications. It is a derivative of the MARC 21 bibliographic format (MAchine-Readable Cataloging) and as such includes a subset of MARC fields, using language-based tags rather than numeric ones. Definitions (semantics) of MODS elements may be found in the appropriate section.

XML Structures

MODS is expressed using the XML (Extensible Markup Language) schema language of the World Wide Web Consortium. XML provides markup for documents and is expected to allow more flexibility and detail than HTML (Hypertext Markup Language). It serves well as a syntax for metadata.

By using the XML schema language, MODS defines main elements, child elements (i.e. subelements), and attributes of elements and subelements. Content of elements are included in the lowest level elements so as to avoid "mixed content", which is when some elements contain character data interspersed with child elements. For instance, if <titleInfo> contains subelements for <title>, <partNumber>, <partName>, then <titleInfo> is only a container tag to include the more specific elements <title>, <partNumber>, <partName> and does not contain any content data. (A "container" tag is one that it used only as an element that binds together child elements, but contains no data other than tags.)

Attributes may be associated with elements at any level and are defined with the element with which they are associated. They serve to modify the element. Common attributes that occur throughout the schema include: type, encoding, and authority.

A MODS document contains a schema declaration that indicates the MODS namespace. Within a record or group of records it is optional to use the "mods" prefix before each element (and before the "mods" namespace declaration), since the MODS namespace is indicated in the record. It is most useful to use the prefix "mods:" before each element when combining a MODS record with XML data from another namespace, e.g. a MODS record within a METS document.

The schema declaration for MODS Version 3 is:

<mods xmlns:mods=""> with filename mods-3-5.xsd

or if using "mods" as a prefix to each element:

<mods: mods xmnls:mods=""> with filename mods-3-5.xsd

An "XSLT stylesheet" (Extensible Stylesheet Language Transformation) may be written to transform the MODS data in some way for output. Examples include using a stylesheet to place the record into a template with easy-to-understand element names in XML; using a stylesheet to formulate a display that looks like a catalog card; using a stylesheet to transform coded data into textual form.

Implementation Notes

Cataloging Rules

Any set of cataloging rules may be used with MODS, as is the case with MARC 21.


If using International Standard Bibliographic Description (ISBD) punctuation in creating MODS records, punctuation should be retained if it occurs within an element, but should be dropped between elements. If desired, an XSLT stylesheet may be used to generate an ISBD display from the MODS elements and data.


<placeTerm type="code" authority="marccountry">nyu</placeTerm>
<placeTerm type="text">Ithaca, NY</placeTerm>
<publisher>Cornell University Press</publisher>

ISBD-based Display after XSLT transformation

Ithaca, NY : Cornell University Press, c1999.

Relationship between MODS and MARC 21 Elements

Most MODS elements have equivalent ones in MARC 21, although there are a few additional elements or attributes in MODS that are particularly important for digital resources, a special target population for MODS. An example is <digitalOrigin> under <physicalDescription>. The MODS Editorial Committee keeps the MARC and MODS element sets in sync as much as possible.

Mapping between MODS and MARC

When using the MARCXML to MODS stylesheet, elements not in the mapping will be dropped and some elements will be mapped to a more general MODS element. Since MODS has fewer data elements than MARC, records converted from MARC to MODS that are then converted back to MARC will lose some data or specificity of tagging.

Order of Elements

The order of elements in the MODS schema does not assume display order. A stylesheet is used to control display order of MODS records.

Element Repeatability

All MODS top-level elements are repeatable except <recordInfo>.

MODS Record Identifier

It is expected that a MODS record would have its own unique ID in addition to any other identifiers associated with the resource and indicated under <identifier>. The MODS record identifier is indicated in <recordInfo><recordIdentifier>.

Root Elements

All top-level MODS elements (those immediately subordinate to <MODS>) are declared as global. Thus, they may be root elements for instance documents, or imported into other schemas. Future work includes specifying some elements at subsequent levels as global.

Mandatory Elements

No element is mandatory in a MODS record, however, every MODS record requires at least one element. Applications may wish to develop profiles specifying mandatory elements as needed. The DLF/Aquifer guidelines specify a profile for sharable metadata in the DLF/Aquifer Summary of Requirements and Recommendations table that indicates required, recommended and optional elements.



Last Updated: July 12, 2013