Serial numbers are unique numbers found on dollar bills. They are 10 digits long on bills made before the 1996 series and 11 digits long on bills made in the 1996 series and after.
On 11-digit bills, the first letter indicates the series of the bill — the year in which the design was approved. For example, a new design is necessary every time the secretary of the treasury changes. The second digit is a letter A through L and indicates which Federal Reserve bank district the bill was printed in. On bills before the 1996 series, this is the first digit in the series. A represents Boston, and L is San Francisco. Eight unique numerical digits follow, and another letter is at the end in case more than 99,999,999 bills need to be printed.