Bank card numbers are made up of different parts, taking the common 19-digit debit card as an example: 1. **Issuing bank identification code**: - This is the first 6 digits of the bank card number, which are assigned to each bank by the International Organization for Standardization (ISO). The combination of these 6 digits is different for different banks and is used to identify the issuing bank. - For example, some of the issuing bank identification codes of Industrial and Commercial Bank of China may be "622202". 2. **Custom digits**: - The next few digits (usually the middle ones) are customized by the bank to distinguish different business types, card types, etc. - For example, for a certain type of savings card, the bank will set a specific combination of digits to identify it. 3. **Check digit**: - The last digit of the bank card number is the check digit. It is calculated using a specific algorithm and is used to verify the legitimacy of the bank card number. - The algorithm for calculating the check digit is relatively complex. In simple terms, it is obtained through a series of mathematical operations based on the previous digits. For example, the first 18 digits may be multiplied by different weight coefficients, and then these products are added together, and then the sum is processed using a fixed rule to obtain the check digit. Credit card numbers are similar in composition, but the number of digits may be different, generally 16 digits, etc. The specific rules are also determined by the issuing institution and relevant standards. The details of bank card numbers in different countries and banks may vary, but generally follow similar coding rules.