The applicant’s SSN is typically written in the upper right corner of the form. The first three digits designate the state or territory where the number was issued. The next pair of digits relate to the sequence in which numbers were issued. The final four digits were assigned chronologically as forms were processed. SSNs beginning with 700 to 728 were used for railroad workers in the Railroad Retirement system. You can “decode” your ancestor’s SSN using Stephen P. Morse’s one-step tool.