What do the letters mean on each dollar bill?

The big letter to the left of Washington's portrait indicates the Federal Reserve branch where the bill was originally distributed. The same letter is found at the beginning of the serial number. The last letter indicates how many runs of serial numbers have been used, where A is the first, B is second, and so on. Then there will be a tiny letter and number, such as J5 or FW D191, which is the plate position during printing.