CSS Position absolute relative to a container

If you are here it's because probably you are looking for the reason why your inner div don't obey to your “right:0” or similar absolute positioning command relative to the upper div.

Say we have double indent,

<div id="a">
	<div id="b"></div>
</div>

#b{position:absolute;right:0}
//it works only if...
#a{position:relative}

So the secret it's just having the container set as position:relative.