Routing transit number
The Routing transit number is a 9-digit number each financial institution in the United States has for use in processing of checks, automated payments, and other inter-bank transactions. It was introduced by the American Bankers Association in 1910, and is used by the Automated Clearing House (not the people who bring oversized checks to your door when you win a sweepstakes; that's Publisher's Clearing House) and the Federal Reserve.
These numbers appear at the bottom of checks (along with account numbers and check numbers) in a 1960s-era magnetic-reading-friendly font that was for many years widely imitated by designers wanting their logos and headlines to look Hi-Tech and Ultra-Modern, but which is actually rather archaic.
The ABA (American Bankers Association, not American Basketball Association or American Booksellers Association) caused some recent controversy by claiming copyright protection to lists of routing numbers and trying to suppress independent projects to distribute such lists in electronic form.