Belt Squeal when A/C on

What causes the belt to squeal when I turn on my A/C?? I just replaced the alternator belt, does the A/C run off of that one or the other one? When I replaced it and tightened the tensioner I didn't know how tight to make it so I just kind of guessed from experience w/ other belts...just in case the tension has anything to do with the squeal. Thanks ahead of time for any help!

I can't remember if the AC belt and the alternator belt are the same but when my car used to sqeak when I had the AC on, I changed the belt and it stopped. If it's different then try changing that.

If the belt is new (and you are sure you changed the right belt) then the tensioner is either too tight or too loose...

Yes, too tight a tensioner will make it squeal as well because then there is too much resistance against the belt and it's being squeezed into the V...

Does it matter that it's not a V-belt? It's the ribbed one, not the V one, that I changed. Anyone know which one the A/C is driven off of?

I have the same problem and i had a guy look at it and he said that the bearing was old and i needed to replace the pully or something along those lines. so yeah i don't know how correct that is but i'd like to get it fixed so any info will help me too.

Go to:
http://www.automotiveforums.com/vbulletin/showthread.php?t=284937&highlight=squeaking+belt
Linebckr49 posted a good picture of the belts and on it it has how tight the belts should be.