Social security number usually refers to social security number, and each country or region has its own unique numbering rules. Here I can introduce the generation rules of social security card numbers for residents in mainland China to help you understand its structure. In mainland China, the social security card number is usually combined with the resident ID number. The structure of the ID number is 18 digits, divided into the following parts: 1. **First 6 digits**: address code, indicating the region where the ID card holder belongs. 2. **Middle 8 digits**: date of birth, format YYYYMMDD. 3. **Last 4 digits**: - The first 2 digits are sequence codes, which are randomly generated. - The third digit is the gender code, which is an odd number for males and an even number for females. - The last digit is a check code, which can be 0-9 or the letter X, and its generation is calculated according to a specific algorithm based on the first 17 digits. Although the social security number and the ID number are combined, in the specific social security field, some regions may add some additional numbers to the ID number to distinguish different social security accounts held by the same ID number. **Example**: Assuming someone's ID number is 110105199001012345, the possible social security number will directly use this ID number or expand on it. Specifically, due to different regions and uses, the detailed social security card number generation rules may be slightly different, but the core is mostly attached to the ID number. Please note: It is illegal to generate or use a false ID number or social security number, and in any case, you should use legally obtained number information.