The card security code is an added safeguard for your credit/debit card purchases. Depending on the type of card you use, it is either a three- or four-digit number printed on the back or front of your credit/debit card, separate from your credit/debit card number. To make shopping at Scientific American Book Club® even more secure, we require that you enter this number each time you make a credit/debit card purchase. Please note that your security code will not be stored with us even if you have saved your credit/debit card information.