In small size currency, prior to the Series 1996 $20, $50 and $100 notes, a note's serial number uniquely identifies that note within the note's type, denomination and series. The serial number on small size currency was standardized to eight digits, with a single letter prefix and single letter suffix. All letters but "O" are used in the prefix and suffix letters - more or less.
The number itself starts with 00000001, and runs through 99999999. What happens next depends on the type of note it is, although in recent decades, there is only one type of note in circulation. But, let's start with the old ones:
Serial Numbers on Silver Certificates, Gold Certificates and United States Notes
For Silver Certificates, Gold Certificates and United States Notes, after the serial number reaches 99999999, the prefix letter is changed. Only after the prefix letter has reached "Z" will the suffix letter change, as illustrated:
Using this numbering scheme, we have 99,999,999 possible digit combinations, by 25 possible prefix letters, by 25 possible suffix letters, producing 62,499,999,375 different combinations. If they used "00000000" it'd be an even 62,500,000,000 combinations, but they don't. Presumably, if this ever happened, they'd have had to start a new series to allow them to reset the serial number back to A00000001A. Fortunately, this never happened.
What did happen was that occasionally, errors would be made in the printing process, and those errors would be caught before released into circulation. Those notes would be destroyed and replaced with a note that had a "*" in place of the prefix letter, and serial numbers starting with *00000001A. Should *99999999A be printed, the next "star note" would continue at *00000001B. This added another 2,499,999,975 possible combinations, which of course, probably never happened.
Serial Numbers on Federal Reserve Notes
With Federal Reserve Notes (the only type we have anymore), the numbering works a little differently. On these notes, the prefix letter corresponds to the Federal Reserve Bank's district. As there are 12 districts, the prefix letter will existing in the range A-L. As of writing, within a Federal Reserve District, the prefix letter is never incremented; after serial number x99999999A, the suffix letter is incremented, as in:
As with the other types of notes, those replaced due to errors in printing contain a "*" in the serial number, this time as the suffix. Thus, for each Federal Reserve Bank, there are 2,499,999,975 possible regular serial numbers, plus 99,999,999 possible star notes. Having recently seen $1 notes in the B..X block, I have to think that this has probably already happened. What they've done about this, I don't know.
The New Stuff
With the introduction of the Series 1996 $20, $50 and $100 notes, another letter was added to the serial number prefix to uniquely identify the note within the note's denomination. This letter is incremented with each series. Thus, serial numbers are eleven characters, starting with the series letter, followed by the Federal Reserve District letter, the eight digit serial number, and single suffix letter.
Other SerialNumber Factoids
Identifying Bogus Error Notes
One interesting factiod concerning recent series of the smaller denominations is that the higher serial numbers (say, x95000000x and up) are reserved for uncut sheets sold by the Treasury. Anyone who wants to can buy one of these, either at the Treasury or by mail. You can paper your wall with them, frame them, or even cut them apart and spend them (although given what the government charges for these sheets, it wouldn't be very cost effective).
Unfortunately, some enterprising miscreants have been buying these things, cutting them apart incorrectly, and selling them as error notes, as if they were miscut currency. Most currency collectors aren't fooled by this, although many other people apparently are.
Sequential Serial Numbers
Another interesting factoid is that since the introduction of the 18 subject sheet, two adjacent notes on an uncut sheet will not bear sequential serial numbers. Serial numbers are in fact assigned sequentially from sheet to sheet, with notes on the same sheet bearing numbers separated by gaps of thousands. This is done so that if one took a stack of one hundred uncut sheets, and bulk cut them, you'd have stacks of 100 sequentially numbered notes without requiring any further sorting. Interestingly, this means that within a block of serial numbers, the higher numbered notes are printed first,so that the lower numbered notes will appear on the top of the stack.
Serial Number Ink Color
You've probably noticed that the serial number is not printed in black
ink. For small size US currency, you can generally determine the type of
note by the color of ink used to print the Treasury seal and serial number (with
some exceptions in emergency currency printed during World War II).
Older, large size currency (series prior to 1928) didn't
conform to any such convention.
Older, large size currency (series prior to 1928) didn't conform to any such convention.