Set of numbers and or letters on a note. The serial number is perhaps one of the oldest security devices on a banknote. Using unique serial numbers on each bill, governments are able to closely track legitimate bills.
Sometimes serial numbers get “unusual” like one number repeated several times or filling series as “12345678” or the notes called “radar” whose serial numbers are the same backward and forward.
Collectors also buy sets of banknotes in consecutive serial numbers.
The notes below show a “radar” serial number 663366 and 6226