An ABA number is officially called an ABA routing transit number (ABA RTN). The ABA routing number, a nine-digit number, includes: • First two numbers: Regional Federal Reserve Bank • Number three: Check processing center • Number four:  Federal Reserve District state • Numbers five through eight: Financial institution identifying number • Number nine: check digit (checksum character) This link to U.S. Bank’s Find Your Routing Number web page includes an ABA number visual example. The fractional ABA number is an eight or nine-digit number that resembles a fraction with a numerator with a hyphen and denominator. According to BankersOnline.com, the fractional numerator is five digits, and the denominator is three or four digits. The first two digits in the numerator before the hyphen are the City/State Prefix, and the three digits after the hyphen are the ABA Institution Identifier. The three or four digits in the denominator are the Federal Reserve Routing Symbol.