The CID is the three- or four-digit security code on your American Express, Diner’s Club, Discover, MasterCard or Visa credit card. In some cases the CID is required to process your order. Locate the CID on your card as shown below:
American Express
Four-digit code located on the front, just above and to the right of your credit card number.
MasterCard
Three-digit code located on the back in the signature box, following the 16-digit credit card number.
Visa
Three-digit code located on the back in the signature box, following the 16-digit credit card number.