Indicates the unique identifier for an associated element, so that the element’s content may be cross-referenced.


For example, when an id attribute has been used on a <glyph-data> element, that identifier can be referenced by an IDREF-style attribute on a Glyph Reference for a Private Character element, so that the full glyph data need not be repeated every time the character is used.


The uniqueness of an identifier is not maintained by the XML; thus, a naming convention should be adopted. This identifier will be referenced by other elements; therefore, it is essential that no logical conflicts occur within a single XML file. An identifier string is typically 1-32 characters in length and starts with a letter of the alphabet. (It is an XML rule that identifiers may not start with a digit.)

Attribute Values

In Elements

<ack> Acknowledgments
<address> Address/Contact Information
<aff> Affiliation
<alt-text> Alternate Title Text (For a Figure, Etc.)
<app> Appendix
<array> Array (Simple Tabular Array)
<author-notes> Author Note Group
<bio> Biography
<boxed-text> Boxed Text
<caption> Caption of a Figure, Table, Etc.
<chem-struct> Chemical Structure (Display)
<chem-struct-wrapper> Chemical Structure Wrapper
<citation> Citation
<col> Table Column (XHTML table model)
<colgroup> Table Column Group (XHTML table model)
<collab> Collaborative (Group) Author
<contrib> Contributor
<corresp> Correspondence Information
<def> Definition List: Definition
<def-item> Definition List: Definition Item
<disp-formula> Formula, Display
<disp-quote> Quote, Displayed
<ext-link> External Link
<fig> Figure
<fig-group> Figure Group
<fn> Footnote
<glossary> Glossary Elements List
<glyph-data> Glyph Data for a Private Character
<graphic> Graphic
<inline-formula> Formula, Inline
<inline-graphic> Graphic, Inline
<inline-supplementary-material> Inline Supplementary Material
<institution> Institution Name: in an Address
<kwd> Keyword
<kwd-group> Keyword Group
<long-desc> Long Description
<media> Media Object
<mml:math> Math (MathML 2.0 DTD)
<named-content> Named Special (Subject) Content
<nlm-citation> NLM Citation Model
<note> Note in a Reference List
<notes> Notes
<p> Paragraph
<preformat> Preformatted Text
<ref> Reference Item
<sec> Section
<speech> Speech
<statement> Statement, Formal
<supplementary-material> Supplementary Material
<table> Table (XHTML table model)
<table-wrap> Table Wrapper
<table-wrap-group> Table Wrapper Group
<target> Target of an Internal Link
<tbody> Table Body (XHTML table model)
<td> Table Data Cell (XHTML table model)
<term> Definition List: Term
<tfoot> Table Footer (XHTML table model)
<th> Table Header Cell (XHTML table model)
<thead> Table Header (XHTML table model)
<tr> Table Row (XHTML table model)
<verse-group> Verse Form for Poetry
<xref> X(cross) Reference
Value Meaning
An identifier (ID) A unique identifier for the element. This can be used by an IDREF-style attribute on, for example, a cross-reference <xref> element, to point to the element that has this ID.
Restriction: This attribute may be specified if the element is used.

In Elements

<def-list> Definition List
<list> List
<list-item> List Item
<tex-math> Tex Math Equation
Value Meaning
Data characters Text, numbers, or special characters.
Restriction: This attribute may be specified if the element is used.