There are typically three codes on the bottom of a check that are used during processing. These codes represent the bank's routing number, the account number from where the funds are being drawn, and the check number.
There are several codes on the bottom of a check written from a U.S. bank. The first code, which is nine digits long on the left side of the bottom of a check, represents the ABA routing code or routing number. The routing number identifies the particular financial institution involved in the financial transaction. Some banks have multiple routing numbers; for example, different branches in a bank institution may have unique routing numbers. The routing number on the check represents the local branch where the debit account number is located.
The second code on a check, usually located in the middle of the bottom line of the check, is the account number that will be debited when the check recipient deposits the check into his bank. The last code on the bottom of the check is the check number. Typically the check number is also listed in the upper right corner of the check.
Since 1959, the numbers on the bottom of physical checks have been read using MICR scanning, or Magnetic Ink Character Recognition. Physical checks have been printed with special ink to absorb and emit a magnetic signal, allowing MICR readers to read the numbers on the checks for quicker processing.