Each XML document has one, and exactly one single root element. This element encloses all following elements and is therefore the parent element to all the other elements. The end-tag of the child element is always before the end-tag of the parent element - this is called nested elements.

<parent>
 <child>
 </child>
</parent>

If the end-tag of a parent element occurs before the end-tag of the child element this is improperly nested and illegal in XML.