For Visa and Mastercard, also known as the Cardholder Verification Value or, CVV2, these three numbers help ensure that the physical card is in the cardholder’s possession while shopping online or by phone, helping to prevent unauthorized or fraudulent use. The 3-digit code is located on the back of your card, inside the signature area. Typically the signature panel will have a series of numbers, but only the last three digits make up the CVV2 code.
For American Express, also known as the Card Identification Digits or, CID. The CID on your American Express Card helps protect you when you shop online or on the phone. This allows American Express to verify that the number given matches your records, helping to protect your account from fraudulent use. The 4-digit code is located on the front of your card, above the account number to the right.