text: The merchant's machine authenticates the account with the financial institution that issued the card. Basically this is checking to see that the card number is valid, and the card itself is valid.\n\nSwiping the card sends the card information, and transaction information to the financial institution. The financial institution returns a numerical code. The code can either be approved, or a trouble code that denies the transaction. If the transaction is denied with a trouble code, the specific code maps to an English definition for the denial of credit. For example, "credit limit exceeded", "card not activated", or "not a valid account".\n\nThe first four-six digits of the account number refer to a specific financial institution, which is a destination for the machine. For example, 5424 is issued by Citibank.