An XML document can use elements declared in different name spaces.

• unless otherwise stated, every element in a document comes from the same default namespace
• the default namespace may be declared using a special xmlns attribute
• other name spaces must all use a special prefix, which is also declared

<TEI xmlns="http://www.tei-c.org/ns/1.0"> ... </TEI>

<TEI xmlns="http://www.tei-c.org/ns/1.0"
            xmlns:math="http://www.mathml.org">
     <p>... <math:expr>...</math:expr> ...</p>
     </TEI>

Multiple schemas may be needed to validate such a document.