The Check Digit The check digit is the last digit of the ISBN, validating the ISBN. It can range from 0 to 9 or be an ‘X’ if the calculated check digit is 10.

Check Digit: The check digit is the last digit of the ISBN. Its value is calculated from the other twelve digits of the ISBN and provides a check on the validity of the ISBN. If a mistake is made in transcribing the ISBN, such as a wrong number or a transposition error, the resulting ISBN will likely be invalid, indicating the error.