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2.6. Namespace URI

Any XML element or attribute may belong to a namespace, that is, a set of elements and attributes that belong to a single document type.

Most XML document types are defined by a schema that not only enumerates all the element and attribute names in that document type, but also defines the rules for structuring the document: which elements may be inside which other elements, and which attributes may occur in which elements.

Throughout the present document, we use the term namespace URI to mean a URI that uniquely identifies a namespace.

For example, the XHTML 1.0 document type is associated with this namespace URI:

In this case, there actually is a DTD (Document Type Definition) schema located on the Web at that URI. However, a namespace URI may be only an identifier, and it may or may not be function as a location.