contact your broker they will explain to you how it works. Remember when you short a stock you have a potential for finite profit but infinite loses that could wipe you out beyond the value of your account. I would not short unless you have 100% confidense in what you are doing and to which stock you are shorting.
Conversly, when you buy a stock and hold it, you have a potential for finite loses if the stock goes to zero, (you lose what you paid for it), but infinite profit if the stock keeps climbing and climbing and climbing. Like FNMA and FMCC are about to do.
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