All DocBook V5.0 elements are in the namespace http://docbook.org/ns/docbook. XML namespaces are used to distinguish between different element sets. In the past few years, almost all new XML grammars have used their own namespace. It is easy to create compound documents that contain elements from different XML vocabularies. Consider this simple article marked up in DocBook V4.5:

<!DOCTYPE article PUBLIC '-//OASIS//DTD DocBook XML V4.5//EN'
                         'http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd'>
<article>
  <title>Sample article</title>
  <para>This is a really short article.</para>
</article>

The corresponding DocBook V5.0 article will look very similar:

<article xmlns="http://docbook.org/ns/docbook" version="5.0">
  <title>Sample article</title>
  <para>This is a really short article.</para>
</article>

The only change is the addition of a default namespace declaration (xmlns="http://docbook.org/ns/docbook") on the root element (and a version attribute, which is described in the next section). This declaration applies the namespace to the root element and all nested elements. Each element is now uniquely identified by its local name and namespace.