The first three digits of a person's Social Security number represent either the area of the country where the number was first issued or the ZIP code of a worker's mailing address. When the Social Security number was created in 1936, the first three numbers - known as the area number - represented the state in which a person first applied for a card, with lower numbers starting in the northeast and moving westward.
The Social Security Board considered different ways of assigning numbers, such as using fingerprints, a person's name or address and various numbering systems consisting of seven, eight and nine digits. After much consideration, a nine-digit system consisting of a three-digit area number, a two-digit group number and a four-digit serial number was put into use.
When Social Security numbers were first issued, the Social Security Board planned to use area numbers to redistribute work to its 12 regional centers, and one or more area numbers were allocated to each state. The board realized, however, that area numbers didn't always reflect a worker's residence. In 1972, the Social Security Administration began issuing numbers centrally from Baltimore, Maryland, based on the ZIP code on the application for an original Social Security card.
There are some exceptions to the general east-to-west ascending order numbering scheme. Some of these include sequences 700 to 728 ,which were assigned to railroad workers until July 1963, and sequence 580, which was assigned to Puerto Rico and the Virgin Islands. No Social Security numbers with the area sequence 666 have been or will be assigned.