Visa, MasterCard, and Discover: 3 digit code on the back of the card in the top-right corner of the signature box. American Express: four-digit code on the front of the card directly above and to the right of the credit card number.
What is a CSN code and where do I find it?
The credit code security code, sometimes known as a CSN code, is a three-digit number printed on the back of your card, at the top of the signature field. It is being increasingly used in online credit card transactions to help prevent fraud.
Visa, MasterCard, and Discover: 3 digit code on the back of the card in the top-right corner of the signature box.
American Express: four-digit code on the front of the card directly above and to the right of the credit card number.