Question: if i place a limit buy order betwen the bid and ask, MUST the bid go up? of can the market maker keep the old? 

Answer: It depends upon the other outstanding bids.\n\nIf it is on the NASDAQ and the bid is 49 and the ask is 49.50 and you offer to buy at 49.25 and you are the only bidder, then you should become the new bid price raising the bid/ask to 49.25 and 49.50.  If there are other bids that may not happen.\n\nIf it is on the exchanges, then you are bidding outside the range.  You will limit the prices when the current best offer is taken, presuming no one else will make an intervening bid.\n\nMarket makers cannot bid against the street, BUT if there are other outstanding orders, they are not bidding against the street.  So they may keep the old if there are outstanding valid prices because your purchase offer is not in the money as the high bid.
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