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Top-level Element: <identifier>

Element <identifier>
Definition Contains a unique standard number or code that distinctively identifies a resource.
Attributes type; displayLabel; invalid; altRepGroup; lang; xml:lang; script; transliteration; typeURI
Subelements None



<identifier> includes manifestation, expression, and work level identifiers.

MODS makes a distinction between a persistent identifier of the resource and its electronic location. This element is used for the former; <location><url> is used for the latter.

Repeat the <identifier> element for each applicable identifier recorded, including invalid and canceled identifiers.

Specific DLF/Aquifer Guidelines

The DLF/Aquifer Implementation Guidelines for Shareable MODS Records recommend the use of at least one <identifier> element with the type attribute containing appropriate values in each MODS record.

The <identifier> element is recommended in these guidelines and refers to the digital resource being described or to its analog equivalent. In the shared metadata context, the <identifier> should identify the object universally.

The guidelines recommend that <identifier> be used to encode any identifying character string uniquely associated with a resource, but reserves <location><url> for encoding the URL to the resource in its repository context. Persistent URIs that serve both as a unique identifier and a URL to the resource in its repository context should be encoded using both elements.

These guidelines require that all <identifier> URLs follow the URI specifications laid out in the DLF/NSDL Best Practices for Shareable Metadata Content.

All occurrences of the <identifier> element must contain the type attribute to indicate the type of the identifier. If a local identifier is used, it should be indicated as such using the "local" value.

See the <identifier> entry in the DLF/Aquifer Summary of MODS Requirements and Recommendations Table for further information on requirements of this element, its attributes, and subelements.

Aggregator information: <identifier> is displayed in some aggregators but is generally not used for retrieval. It may in the future be used to de-duplicate aggregations.

Identifier is addressed in the "Recommendations for classes of data elements" under "Identifiers." The DLF/Aquifer guidelines recommend the use of the type attribute in lieu of prefixing identifiers, as recommended in DLF/NSDL Best Practices for Shareable Metadata External Link.

Element Description



Identifies the type of identifier that is recorded.

There is no controlled list of identifier types. Suggested values include, but are not limited to, those specified in Standard Identifier Source Codes.


Used to indicate that a type is in the form of a URI.
URIs identifying authorities may or may not be dereferenceable to human- or machine-readable information on the authority file, controlled vocabulary, or thesaurus.


Signifies a canceled or invalid identifier.
The invalid attribute should be used with the following value:
  • yes – This value is used only when recording a canceled or invalid identifier. If invalid="yes" is not present, the identifier is assumed to be valid. Do not use invalid="no."

altRepGroup; lang; xml:lang; script; transliteration; typeURI; displayLabel

See the Attributes used throughout the schema for descriptions of each.


There are no subelements for <identifier>.



<identifier type="music publisher">N.M. 275 Nova Music</identifier>
<identifier type="lccn">##2001336783</identifier>
<identifier type="doi">doi:10.1006/jmbi.1995.0238</identifier>
<identifier type="hdl">hdl:loc.pnp/cph.3c30104</identifier>
<identifier type="uri"></identifier>
<identifier type="isbn" invalid="yes">0877780116</identifier>
<identifier type="uri" invalid="yes"></identifier>
<identifier type="uri" displayLabel="Active site"></identifier>
<identifier type="uri" displayLabel="Essays from annual reports"></identifier>



MARC Mapping (Bibliographic)

<identifier> ≈ MARC 21 fields 010, 020, 022, 024, 856 (only when string begins urn:hdl, urn:doi, hdl: or doi:)

See also MARC Mapping to MODS for the <identifier> element.

Dublin Core Mapping

The MODS <identifier> element, along with the <location><URL>, are both mapped to <dc:identifier> in the MODS to Dublin Core Metadata Element Set Mapping.

MODS examples expressed in Dublin Core:



Last Updated: August 2, 2018