A Social security number (SSN) consists of nine digits, commonly written as three fields separated by hyphens: AAA-GG-SSSS. The first three-digit field is called the "area number". The central, two-digit field is called the "group number". The final, four-digit field is called the "serial number".
The process of assigning numbers has been changed at least twice. Until 1965, only half the group numbers were used. Before 1972, numbers were assigned by field offices; since 1972, they have all been assigned by the central office. The order in which numbers were assigned was changed in the 1972 transition. There may have been other changes, but it's difficult to get information on how things used to be done.
Any SSN conforming to one of the following criteria is an invalid number:
A pamphlet entitled "The social security number" (Pub. No. 05-10633) provides an explanation of the SSN's structure and the method of assigning and validating social security numbers.
This description of the structure of the social security number is based on messages written by Jerry Crow and Barbara Bennett. The information has been verified by its correspondence to the SSA's Program Operations Manual System (POMS) Part 01, Chapter 001, subchapter 01, which can be found at Federal Depository Libraries. (SSA Pub. No. 68-0100201.)
