What does an underscore “_” mean in CSS?

Question

I just found the following snippet in a CSS file:

position: fixed;
_position: absolute;

What does this underline mean in front of the second

Answer 1

This is an old CSS-Hack for IE5, 5.5 & 6.
All browser will display the position:fixed while IE5 - 6 use the _position, so it display it absolute.

But note: This CSS won't validate! And it won't work for IE5/MAC