Nesting Lists 

Nested lists are used when using both unordered and ordered lists. A nested list is a list within a list. To create a correctly nested list in XHTML, the trick is to be sure the nest is contained completely within an open and closing list item element

Eample 

<ol>
<li>IBM</li>
<li>SUN</li>
<li>MICROSOFT
     <ol>
     <li>XP </li>
     <li>2000 </li>
     <li>98</li>
     </ol>
</li>
</ol>