The last four digits of a Social Security number are called the serial number. The numbers that can be used as the last four numbers of a Social Security number run consecutively from 0001 to 9999 and are issued in accurate sequence depending on when the Social Security number request is received.
The first three numbers of a Social Security number are indicative of the location of the person. Before the Social Security offices moved to Baltimore in 1972, people were able to apply for a Social Security card at local offices. The office where they applied was the factor that dictated the first three numbers. Since the offices are centralized in Baltimore, the location number is now chosen according to the ZIP code that is listed on the application. The lowest numbers are on the East Coast while the highest are on the West Coast.
The number in the middle of the Social Security number is related to the group that the state is within. These numbers are not issued in sequence and the odd numbers are issued first. When the maximum number of odd numbers is reached for a particular group, the even numbers will then be used. A person who applies for a Social Security number immediately after another person may have a completely different group number than the person before them.