From the U.S. Navy to private closet

The first turning point in history of the T-shirt was when, at the beginning of the 20th century, it became an official undergarment of the U.S. Navy. A crew-necked, short-sleeved, white piece of clothing was supposed to be worn under a uniform. In informal circumstances, however, sailors and Marines used to take the top parts of their uniforms off and work having only undershirts on. Due to how comfortable and practical the short-sleeved undergarment was a lot of Marines started to use it off-duty as an every-day piece although it was not meant to be worn in that way.