Since 1911, the American Bankers Association has partnered with a series of registrars, currently Accuity, to manage the ABA routing number system.[2] Accuity is the Official Routing Number Registrar and is responsible for assigning ABA RTNs and managing the ABA RTN system. Accuity publishes the American Bankers Association Key to Routing Numbers semi-annually. The “Key Book” contains the listing of all ABA RTNs that have been assigned.

There are approximately 26,895 active ABA RTNs currently in use.[3] Every financial institution in the United States has at least one. The Routing Number Policy allows for up to five ABA RTNs to be assigned to a financial institution. Many institutions have more than five ABA RTNs as a result of mergers.

ABA RTNs are only for use in payment transactions within the United States. They are used on paper check, wire transfers, and ACH transactions. On a paper check, the ABA RTN is usually the middle set of nine numbers printed at the bottom of the check. Domestic transfers that use the ABA RTN will usually be returned to the paying bank.

Incoming international wire transfers also use a BIC code, also known as a SWIFT code, as they are administered by the Society for Worldwide Interbank Financial Telecommunication (SWIFT) and defined by ISO 9362. In addition, many international financial institutions use an IBAN code.

The IBAN was originally developed to facilitate payments within the European Union but the format is flexible enough to be applied globally. It consists of an ISO 3166-1 alpha-2 country code, followed by two check digits that are calculated using a mod-97 technique, and Basic Bank Account Number (BBAN) with up to thirty alphanumeric characters. The BBAN includes the domestic bank account number and potentially routing information. The national banking communities decide individually on a fixed length for all BBAN in their country.

https://dev.to/routingnumber/how-to-recognize-fractional-routing-numbers–meb

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