What makes 'cos' unique?

Note: the category and title are both clues and you may need to use some twisted logic as the answer is not straightforward

(In reply to Good puzzle by Tristan)

I'm picky about this, but 's' is a bit straight in the middle as well.
But I think it's the solution to this problem.

Posted by abc on 2003-09-23 07:38:45