Four-digit number above and to the right of the embossed card number on the front of the card
VISA, MasterCard, Discover:
Three-digit number at the top-right corner of the signature strip on the back of the card
How will the card identification number protect me?
By asking you for your card identification number, we are taking an extra precaution to protect you against fraud. Credit card companies incorporate these numbers for situations where merchants cannot visually verify that you have the credit card in your possession, such as Internet transactions.