The series number that appears twice on the front face of a dollar bill helps the Federal Reserve keep track of how many of these bills circulated. The series number will never be exactly the same on two different bills.
The letters A through L start off every series number. Each letter corresponds to one of 12 Federal Reserve districts—among them, Atlanta, Boston, Dallas, Philadelphia, San Francisco and New York—and is a record of where the bills were printed.
Eight numbers appear between each starting and ending letters, and 832 bills are printed with each serial number. Each number is made distinct with prefix and suffix letters.
Series numbers end with a second letter, immediately following the eight numeral digits. This suffix letter indicates the number of times the Bureau of Engraving and Printing used this sequence of serial numbers. A is the first time, B is the second time and so on.
There is also a series date, listed to the bottom right of the U.S. president’s photo. It is marked "Series," followed by a year. The printed year tells which year the particular bill's design was implemented when the bill went into circulation.
Occasionally during the printing process, bills can become mangled. Instead of reprinting the bill using the originally issued serial number, a new bill is printed with a star replacing the suffix letter.