stacking filters?

[Fotografer]

How is it done, and why is it done? With what filters is it done? </p>
<p>&nbsp;</p>
<p>I am finally finally <i>finally</i> getting a polarizing filter soon, something I've been dithering over for most of this year. <img src="http://thephotoforum.com/forum/images/smilies/048.gif" border="0" alt="" title="er" smilieid="109" class="inlineimg" /> I'd like to understand these things.</p>
<p>&n