Today, the T-Shirt is one of the most popular pieces of clothing most people have. Once designed as an undershirt in Europe, the T-shirt became quickly more than just a piece that has to be worn under a shirt.
During WWI the American troops were pleased to see that German and other troops wore obviously something that was lighter and more breathable than their all-season woolen underwear. It seemsed to be the right undershirt to wear during summer.
They brought the idea back home and it was put into practice. During WWII the T-Shirt, named after the shape it has, was standard gear for all soldiers.
Since it was comfortable it became more and more a piece of clothing that was worn not only under a shirt but as the main shirt.
Apparently it looked cool and masculine and in 1955 the worlwide breakthrough of the T-Shirt began when the buff body of James Dean was seen in the movie 'Rebel without a Cause'. The T-Shirt became the number one (upper) piece for casual wear among teenagers, twens, and everybody who liked to make a 'statement', some say it was the 'statement of a rebellious youth'. Trousers-wise the Jeans completed the look and this kind of a casual wear uniform became worldwide number one among young and old.
The 70's brought the Flower Power movement and T-Shirts became quite colorful. It's absolutely a statement when you wear such a T-Shirt nowadays!
Another trend was to print a phrase, a philosophy of life, or funny or rather offensive remarks on the T-Shirt - again... to express the respective person's spirit.