Before electronic markets, there were people who acted as market makers. I understand that they made lots of money because that could buy at the bid and sell at the ask. My question may be naÃ¯ve but with all electronic markets, do the exchanges now make the money on the bid/ask spread that used to be made by the market makers? Further, is there any inherent reason, other than to make money, for the exchanges to have separate bid and ask prices? Could there just be one price?