"Dog tags" is an informal but common term for the identification tags worn by military personnel. The tags are primarily used for the identification of dead and wounded soldiers; they have personal information about the soldiers and convey essential basic medical information, such as blood type and history of inoculations. The tags often indicate religious preference as well. Dog tags are usually fabricated from a corrosion-resistant metal. They commonly contain two copies of the information, either in the form of a single tag that can be broken in half or two identical tags on the same chain. This duplication allows one tag (or half-tag) to be collected from a soldier's body for notification and the second to remain with the corpse when battle conditions prevent it from being immediately recovered. The term "dog tags" arose because of their resemblance to animal registration tags.

A type of dog tag ("signaculum"), was given to the Roman legionnaire at the moment of enrolment. The legionnaire "signaculum" was a lead disk with a leather string, worn around the neck, with the name of the recruit and the indication of the legion of which the recruit was part. This procedure, together with enrolment in the list of recruits, was made at the beginning of a four-month probatory period ("probatio"). The recruit got the military status only after the oath of allegiance ("sacramentum"), at the end of "probatio", meaning that from a legal point of view the "signaculum" was given to a subject who was no longer a civilian, but not yet in the military.

In more recent times, dog tags were provided to Chinese soldiers as early as the mid-19th century. During the Taiping revolt (1851–66), both the Imperialists (i.e., the Chinese Imperial Army regular servicemen) and those Taiping rebels wearing a uniform wore a wooden dog tag at the belt, bearing the soldier's name, age, birthplace, unit, and date of enlistment.

Source: Dog tag from Wikipedia

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