# Structure and generation rules of Social Security Number (SSN) The U.S. Social Security number is a nine-digit identifier, and the format is usually expressed as: XXX-XX-XXXX. Its structure and generation rules are as follows： ## Structure division 1. **Area code** (the first three digits): Historically represents the issuing area, and was allocated by geographic area before 1972 2. **Group number** (middle two digits): Group the numbers in the area 3. **Serial number** (last four digits): the serial number in each group ## Evolution of generation rules -Initial design in 1936: system allocation based on geographic area and group -After 1972: The centralized issuance system was adopted, and the geographical significance was no longer strictly followed. -After 2011: use of "randomized distribution method" to eliminate geographic links ## Explanation of sample number The following are some fictitious examples (non-real and valid SSNs)： **Example 1**: 123-45-6789 -Area code (123): Under the old system, it may represent New York State -Group number (45): specific group identification -Serial number (6789): the sequence number in this group **Example 2**: 540-21-8976 -Area number (540): Under the old system, it may belong to the distribution of the western region -Group number (21): The group number of the area -Serial number (8976): The serial number inside the group Please note: Social Security numbers are extremely sensitive personally identifiable information, and in fact, your real SSN should never be shared. The above example is for teaching purposes only and is not a valid number.