Banks in the United States conform to a numbering system to allow for electronic transfer of funds between different banks. As all money is physically held and transferred by the Federal Reserve Banks, they maintain the system itself.
On paper checks, this number is visible on the bottom, along with the account number and check numbers in OCR format.
A file containing a list of current banks can be downloaded from:
It is in a fixed-width positional format, with the following pattern:
Knowledge of this system is useful in validating payment information entry.