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A Card Verification Number (CVN) is a 3 or 4 digit number printed on a credit card. It provides extra security for card purchases on the Internet or over the phone.

MasterCard and Visa:  The 3-digit CVN is on the back of the card, to the far right of the signature box.

American Express: The 4-digit CVN is on the front of the card, above the account number.