Visa's card number is 16 digits, usually starting with "4". The specific card number is stamped on the card and can be seen on the front of the card.

1-6 digits belong to BINNO issuing bank. 7- 12 is the card serial number; 13 represents the primary and secondary cards, 0 represents the primary card, and 1-9 represents the secondary card; The date when the 14 representative statement was issued; 15 represents which card this card is; 16 bits are check codes.

The number visa itself is not specified, but is defined by each card-issuing bank, so different card-issuing banks have different definitions of the number 13.