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.

Area Numbers

The area numbers are assigned to geographical locations. They were originally assigned the same way that zip codes were later assigned (in particular, area numbers increase from east to west across the continental US as do the ZIP codes). Most area numbers were assigned according to state (or territorial) boundaries, although the series 700-729 was assigned to railroad workers regardless of location (this series of area numbers was discontinued in 1964 and is no longer used for new SSNs). Area numbers assigned prior to 1972 are an indication of the SSA office which originally issued the SSN. Since 1972 the area number in SSNs corresponds to the residence address given by the applicant on the application for the SSN.

In many regions the original range of area number assignments was eventually exhausted as population grew. The original area number assignments have been augmented as required. All of the original assignments were less than 585 (except for the 700-729 railroad worker series mentioned above). Area numbers of "000" have never been issued.

The following table is now out of date. The SSA currently maintains an up-to-date version. I'm leaving the old one here in case the SSA's version goes away. As of 2001/5/15, the SSA lists group numbers as high as 768 as having been issued.

  001-003 NH    400-407 KY    530     NV 
  004-007 ME    408-415 TN    531-539 WA
  008-009 VT    416-424 AL    540-544 OR
  010-034 MA    425-428 MS    545-573 CA
  035-039 RI    429-432 AR    574     AK
  040-049 CT    433-439 LA    575-576 HI
  050-134 NY    440-448 OK    577-579 DC
  135-158 NJ    449-467 TX    580     VI Virgin Islands
  159-211 PA    468-477 MN    581-584 PR Puerto Rico
  212-220 MD    478-485 IA    585     NM
  221-222 DE    486-500 MO    586     PI Pacific Islands*
  223-231 VA    501-502 ND    587-588 MS
  232-236 WV    503-504 SD    589-595 FL
  237-246 NC    505-508 NE    596-599 PR Puerto Rico
  247-251 SC    509-515 KS    600-601 AZ
  252-260 GA    516-517 MT    602-626 CA
  261-267 FL    518-519 ID    627-645 TX
  268-302 OH    520     WY    646-647 UT
  303-317 IN    521-524 CO    648-649 NM
  318-361 IL    525     NM    *Guam, American Samoa, 
  362-386 MI    526-527 AZ     Philippine Islands, 
  387-399 WI    528-529 UT     Northern Mariana Islands

  650-699 unassigned, for future use
  700-728 Railroad workers through 1963, then discontinued
  729-799 unassigned, for future use
  800-999 not valid SSNs.  Some sources have claimed that numbers
          above 900 were used when some state programs were converted
          to federal control, but current SSA documents claim no
          numbers above 799 have ever been used.

As of Feb 10, 1999 the most recent area numbers to have been assigned include 650-658, 667-675, and 680. This list is from the SSA's web site, which shows the highest group number assigned for each area.

Group Numbers

The group number is not related to geography but rather to the order in which SSNs are issued for a particular area. Before 1965, only half the group numbers were used: odd numbers were used below 10 and even numbers were used above 9. In 1965 the system was changed so assignments continued with the low even numbers and the high odd numbers. So, group numbers for each area number are assigned in the following order:

1. Odd numbers, 01 to 09
   
2. Even numbers, 10 to 98
   
3. Even numbers, 02 to 08
   
4. Odd numbers, 11 to 99

In each region, all possible area numbers are assigned with each group number before using the next group number. This means the group numbers can be used to find a chronological ordering of SSNs within a region. When new group numbers are assigned to a state, the old numbers are usually used up first.

SSA publishes a list every month of the highest group assigned for each SSN Area. For example, if the highest group assigned for area 999 is 72, then we know that the number 999-04-1234 is an invalid number because even Groups under 9 have not yet been assigned.

Serial Numbers

Serial numbers are assigned in chronological order within each area and group number as the applications are processed. Serial number "0000" is never used. Before 1965, when number assignment was transferred from field offices to the central office, serial numbers may have been assigned in a strange order. (Some sources claim that 2000 and 7000 series numbers were assigned out of order. That no longer seems to be the case.) Currently, the serial numbers are assigned in strictly increasing order with each area and group combination.

Invalid SSNs

Any SSN conforming to one of the following criteria is an invalid number:

1. Any field all zeroes (no field of zeroes is ever assigned).
2. First three digits above 770

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.)

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