A routing transit number (RTN) is the formal name for a US bank routing number, the nine-digit code that identifies the financial institution on which a payment is drawn or to which it is sent; the main entry explains how routing numbers are structured and used. The longer name comes from cheque processing, where the number was printed so that items could be routed in transit between banks.
The terms routing number, routing transit number and ABA number generally refer to the same identifier, which is administered under the authority of the American Bankers Association. It appears in the MICR line at the bottom of a cheque, is entered when setting up direct deposits and ACH payments, and is used for domestic wires. The final digit is a check digit calculated from the first eight, which helps catch typing errors.
Some banks publish different routing numbers for different services, for example one for ACH and another for wires, so payers should use the number the bank specifies for each payment type. Routing transit numbers identify only US institutions; international payments use codes such as SWIFT BICs or IBANs instead.