Most people recognize this as a “routing number.”
The American Bankers Association (ABA) assigns a number to each banking institution registered with them, for the purposes of electronic transfers; this is commonly known as a routing number, and officially an RTN, or Routing Transit Number.
Consumers and bank clients are familiar with this number as the 9-digit number that appears beside their account number on the bottom of their personal checks. Every client with a particular bank will have have the same routing number on their checks.
The ABA started using these numbers as early as 1910, and it identified the origin or destination of payments to and from financial institutions by clearing houses. ABA member banks must also be approved to hold an account at a Federal Reserve Bank.
The “banks”referred to here can also include credit unions and state-chartered banking institutions. Certain numbers in the beginning of the RTN denote certain types of institutions and certain geographical areas, similar to the Country Code and Area Code that precede the 7-digit local part of a telephone number.