Short Rules of XML

If you're not familiar with it, upon encountering Magento's XML-based layout updates, you may have a question or two about the rules you must follow when modifying an XML file.
Only two set of rule need to remember with XML  is that when a tag opens, it must either be followed by a
closing tag (<xml_tag></xml_tag>) or 
self-close tag (<xml_tag/>) exactly as (X)HTML file tags would.