Structure of Social Security Numbers

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.

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