A nine-digit number identifying a specific depository institution and appearing on all of its cash items. These numbers are assigned by Thomson Directory and are listed in its annual publication, Key to Routing and Transit Numbers.
Source: Federal Reserve Bank
American Bankers Association Number - often referred to as the "transit routing number", is the nine (9) digit electronic address of a financial institution. The ABA number is encoded in the MICR (Magnetic Ink Character Recognition) line of all checks, and is assigned to each financial institution and each branch office of that financial institution.
Source: U.S. Patent and Trademark Office.