A social security number is required when applying for a job or collecting social security benefits. Because it is closely tied to personal information, each person's social security number has specific meaning and is not assigned arbitrarily.
A social security number is a unique identifier that you keep for life, in spite of changes in name and circumstances.
The nine-digit social security number consists of three different parts: the area number, the group number and the serial number.
The first set of three numbers refers to the area number. These numbers are assigned according to the zip code used on the application. However, area numbers prior to 1972 were assigned according to state.
The middle two digits of the social security number refer to a group number, with everyone assigned into an "area" of smaller groups. These two numbers are not assigned consecutively. Odd numbers are first used, then even numbers.
The four-digit serial number distinguishes individuals within a group. These are assigned in order from 0001 to 9999.