Indicates the unique identifier for an associated element, so that the element’s content may be cross-referenced. IDs must be unique inside their document (not just within their element type), and the XML parser enforces this.


The ID and IDREF attribute values constitute a primitive reference mechanism for XML. For example, when an id attribute has been used on a <page-start> element, that identifier can be referenced by an IDREF-style attribute on a cross-reference <xref> element, so the page start can be pointed to.


An identifier string is typically 1-32 characters in length and must start with a letter of the alphabet. (It is an XML rule that identifiers may not start with a digit.)

Attribute Values

In Element

<page-start> Page Start Information
Value Meaning
An XML identifier (ID) A unique identifier for the element. An XML parser can verify that an IDREF-style attribute pointing to one of these is pointing to a valid ID.
Restriction: This attribute may be specified if the element is used.