ABA routing number
Nine digits that identify an American bank in the cheque, ACH and Fedwire systems.
The American Bankers Association introduced the numbers in 1910, when cheques were sorted by hand and someone needed to know which bank a piece of paper should be sent to. The format has outlived a century: four digits of Federal Reserve routing symbol, four for the institution, and a check digit computed with the weights 3, 7, 1 repeated across all nine. It sits at the bottom left of every cheque, printed in magnetic MICR ink.
The same numbers now drive ACH and Fedwire, though not always the same number for the same bank – plenty have one for cheques and another for wires. The US never adopted IBAN, so anyone receiving dollars from abroad still hands over a routing number, an account number and a BIC, and hopes the intermediaries read them in the right order.