How is the verification number used?

For your safety and security, we require that you enter your credit card's verification number. If you are using Visa or MasterCard : The verification number is a 3-digit number printed on the back of your card. It appears after and to the right of your credit card number. If you are using American Express: The American Express verification number is a 4-digit number printed on the front of your card. It appears after and to the right of your credit card number.