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A simple example would be to consider an XML instance that contained references to a customer and an ordered product. Both the customer element and the product element could have a child element "ID_number". References to the element ID_number would therefore be ambiguous unless the two identically named but semantically different elements were brought under namespaces that would differentiate them.
A namespace is declared using the reserved XML attribute <code>xmlns</code>, the value of which must be a [[Uniform Resource Identifier|URI]] (Uniform Resource Identifier) reference, e.g.
<pre><nowiki>xmlns="http://www.w3.org/1999/xhtml"</nowiki> Note, however, that the URI is not actually read, it is simply treated by an XML parser as a string. For example, [http://www.w3.org/1999/xhtml http://www.w3.org/1999/xhtml] itself does not contain any code, it simply describes the [[xhtml]] namespace to human readers. Using a URI (such as <nowiki>"http://www.w3.org/1999/xhtml"</nowiki>) to identify a namespace, rather than a simple string (such as "xhtml"), reduces the possibility of different namespaces using duplicate identifiers. The declaration can also include a short prefix with which elements and attributes can be identified, e.g.
An XML namespace does not require that its vocabulary be defined, though it is fairly common practice to place either a [[Document Type Definition]] (DTD) or an [[XML Schema]] defining the precise data structure at the ___location of the namespace's URI.
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