An ABA number, also known as a routing number or routing transfer number, is a sequence of nine numeric characters used by banks in the United States to identify specific financial institutions responsible for the payment of a negotiable instrument. The ABA routing number was developed by the American Bankers Association in 1910 and has evolved over the years to designate participants in automated clearinghouses, electronic funds transfer, and online banking.

The ABA routing number is printed on the bottom of checks and is the left-most number, followed by the account number, and then by the number of the check. It can also be found by logging into your account and selecting the desired account, then selecting the Account options link (below the available balance), and then selecting Account information.

An ABA routing number can be issued only to a federal or state-chartered financial institution that has been determined to be eligible for a master account by a Federal Reserve Bank. The assignment of a routing number does not expand or change in any way the powers of the financial institution as specified in its charter.

The ABA routing number is used for payment transactions within the United States, including paper checks, wire transfers, and ACH transactions. It helps banks in the process of transferring money domestically between accounts for different kinds of transactions, such as direct deposit or bank transfers.

In summary, an ABA number is a nine-digit code used by banks in the United States to identify specific financial institutions responsible for the payment of a negotiable instrument. It is used for payment transactions within the United States, including paper checks, wire transfers, and ACH transactions.