United States dollar
To meet the demands of their customers, banks get cash from Federal Reserve Banks. Most medium- and large-sized banks maintain reserve accounts at one of the 12 regional Federal Reserve Banks, and they pay for the cash they get from the Fed by having those accounts debited. Some smaller banks maintain their required reserves at larger, "correspondent," banks. The smaller banks get cash through the correspondent banks, which charge a fee for the service. The larger banks get currency from the Fed and pass it on to the smaller banks.