The three-digit number on the back of a credit card, often called the CVV2 code or security code, is an important part of the credit card and can only be known by the issuing bank and the cardholder. This number is not directly displayed on the card like the validity period, but is generated through a complex algorithm, which together with the card number and validity period ensures the security of online transactions.

The CVV2 code is vital to protecting the financial security of cardholders as it plays a key role in online payments that do not require a password. However, due to its easily overlooked nature, users need to be extra vigilant to avoid leaks. Many overseas websites only require the card number, validity period and CVV2 code when making payments, so it is crucial to protect this number.

It is worth noting that different credit card brands have different names and locations of security codes. For example, UnionPay's CVN2 is in the last four digits of the card number in the signature column, MasterCard's CVC2 is also in the same position, and DiscoverCard's security code is called CardmemberID, which is located in the signature column. The CID of the American Express card is printed above the credit card number on the front, while the CAV2 of the JCB card is printed on the last four digits of the card number in the signature column on the back. Therefore, it is each cardholder's responsibility to know and properly keep their respective card's security code.

In general, mastering the method of querying the three-digit security code on the back of a credit card will not only help protect personal information, but also effectively prevent fraud. Be sure to treat it with caution and keep it properly.