QNames were introduced by XML Namespaces in order to be used as URI references. QName stands for "qualified name" and defines a valid identifier for elements and attributes. QNames are generally used to reference particular elements or attributes within XML documents.
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Motivation
Since URI references can be long and may contain prohibited characters for element/attribute naming, QNames are used to create a mapping between the URI and a namespace prefix. The mapping enables the abbreviation of URIs, therefore it achieves a more convenient way to write XML documents. (see Example)
Formal definition
QNames are formally defined by the W3C as:
NCName is defined as follows:
Whereby the Prefix is used as placeholder for the namespace and the LocalPart as the local part of the qualified name. A local part can be an attribute name or an element name.
Example
In line two the prefix "x" is declared to be associated with the URI "http://example.com/ns/foo". This prefix can further on be used as abbreviation for this namespace. Subsequently the tag "x:p" is a valid QName because it uses the "x" as namespace reference and "p" as local part. The tag "doc" is also a valid QName, but it consists only of a local part.