In military circles, dog tags are the regulation identification tags issued to all active duty soldiers, although they are now formally called "I.D. tags" to avoid any derogatory implications. Military personnel are required to keep two sets of I.D. tags around their necks while in uniform, one on a neck-length chain and another on a shorter chain attached to the first. The longer set remains with an injured or deceased soldier, while the shorter set is sent off to the hospital or graves registration unit for processing.
The unofficial name for these I.D. tags comes from their resemblance to actual tags used for identifying dogs. Modern tags are generally made from aluminum blanks fed through a metal embossing machine. Vintage ones from the 1940s through the early 1970s had a notch on one side, but modern dog tags are completely smooth. Advances in identification technology now give military officials the ability to store all of a soldier's medical records on a small electronic chip contained in the tags.
Dog tags were not always standard military gear, however. During the Civil War, the identification of individual soldiers lost on a large battlefield was nearly impossible. Some soldiers would sew strips of cloth with personal identification to the backs of their uniforms, or else purchase special commemorative pins stamped with their name and regiment. At that time, the United States military had no established policy on the identification of soldiers, although there were several purveyors of commemorative pins who offered their services to the government.
It wasn't until 1906 that military regulations were changed to require a standard set of identification tags, and the system of wearing two separate sets of tags only became required in 1916. Dog tags issued by the quartermaster's office included the soldier's full name (in reverse order), Social Security number, military service number, blood type and religious affiliation. Older tags may also include information on a soldier's tetanus shot history.
The notch contained in sets of dog tags has long been a source of controversy. Some believed it was created to accommodate the front teeth of deceased soldiers as the tags were placed in their mouths. The notch allegedly made it easier for other soldiers to forcibly maneuver them into the proper position. Others suggested the notch held a deceased soldier's mouth open to prevent a dangerous build-up of internal gases. There was also a rumor that it indicated the position of the first nail in a deceased soldier's coffin.
Ironically, one of the leading theories debunking these battlefield myths is, in fact, a myth itself. Allegedly, the notch in standard dog tags was created to align the metallic blank in the embossing machine properly, and it promptly disappeared when more modern embossing equipment became available. In reality, the notch had nothing to do with the creation of a tag. Whenever military medics needed to transfer a soldier's dog tag information to official paperwork, they used a machine called the Addressograph Model 70. This machine had a slot for the proper placement of the tags while the embossed side was inked and pressed onto the paper. The notch ensured that the I.D. would be oriented correctly, since they would not fit into the machine in any other position.