One of the first orders out of General Headquarters, American Expeditionary Forces (AEF), Paris, in 1917 was the use of identity tags by all members of the AEF. The tags, also called identity discs, were to be made of aluminum the size of silver half dollars with the information about the individual stamped on them. Two tags were issued to each soldier; in the event of death, one tag was left with the body in burial and the other tag was to be removed and sent to the Chief of the Burial Department. World War I was the first time identity discs were issued on a grand scale and by general order of the headquarters.
Each soldier was to be assigned a service number. It was a part of his official designation, never changed and never reassigned to another soldier. Along with the number, the early discs were stamped with the name, rank, company, and regiment. A later change to the identity tags eliminated all unit references for security reasons, especially for soldiers near or in the front lines. This was done to give less information to the enemy if the soldier was captured.
On request by the wearer, the letter C, H, or P (standing for Catholic, Hebrew, or Protestant) could be added. The discs were worn on a cord around the neck. Alternatively, the Navy generally used an oval tag on a wrist chain. The term “dog tag” for identity discs came into general use after World War I.
Source: WWI Dispatches from the Front, National WWI Museum, Fall 2012