The signature panel code (SPC) is a security code printed on your card. It's used in connection with your card number and expiration date when making certain transactions. 

Visa and Mastercard
The code is three digits long and found on the signature strip on the back of the card.

American Express
The code is four digits long and found on the front of the card, next to the card number.

Additional information:

A signature panel code is known by several names. Here's a brief list of terms that describes this code:

• CVV: Card verification value
• CVN: Card verification number
• CVD: Card verification data
• CVC: Card verification code
• CSC: Card security code