U.S. Army serial numbers, also known as service numbers, were issued to all members of the U.S. Army from 1918 to 1969. Each number uniquely identified an individual service member.
After 1969, the U.S. Army discontinued the issue of service numbers, opting instead to identify soldiers by Social Security numbers. This left nearly 20 million service numbers unused.
One can identify the time period in which a service member enlisted or was drafted based on the service number issued. The numbers also indicate in which state a person entered service. Prefixes were used to make various distinctions, such as whether a service member enlisted voluntarily or was drafted and whether a member was part of the National Guard, Army Reserves or the regular Army.