The ABA routing number is a 9-digit identification number assigned to financial institutions by the American Bankers Association (ABA). This number identifies the financial institution upon which a payment is drawn. Routing numbers are sometimes referred to as check routing numbers, ABA numbers or routing transit numbers (RTN). Routing numbers may differ depending on the state in which the beneficiary account was opened and the type of transaction being made. You may obtain the ABA number from the beneficiary bank's website.