What geographical information was used in alloying the first three digits of social security numbers to the state

Since 1973, social security numbers have been issued by our central office. The first three (3) digits of a person's social security number are determined by the ZIP Code of the mailing address shown on the application for a social security number. Prior to 1973, social security numbers were assigned by our field offices. The number merely established that his/her card was issued by one of our offices in that State. See also High Group List of SSN's.