Default Namespaces : Namespace « Namespace « XML Tutorial

A default namespace is like a regular namespace.
You don't have to specify a prefix for all of the elements that use it. 

<person xmlns=""> 

You can declare more than one namespace for an element, but only one can be the default. 

<person xmlns="" 
    <xhtml:p>This is XHTML</xhtml:p> 

2.1.1.Why We Need Namespaces
2.1.2.How XML Namespaces Work
2.1.3.Default Namespaces
2.1.4.Declaring Namespaces on Descendants
2.1.5.Understanding URIs
2.1.6.Change URI of namespace
2.1.7.Using Prefixes