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.

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.
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.

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:
Odd numbers, 01 to 09
Even numbers, 10 to 98
Even numbers, 02 to 08
Odd numbers, 11 to 99
Group codes of "00" aren't assigned

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:
Any field all zeroes (no field of zeroes is ever assigned).
First three digits above 740

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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