\A T-shirt is a wardrobe item that originally belonged to underwear and was worn under a shirt. This same garment is called an undershirt or sweatshirt and continues to be produced at the present time. Often, for underwear, a slightly ribbed fabric and a triangular collar are used so that the underwear is not visible when the top 1 or 2 buttons on the shirt are unbuttoned. The fabric in this case is either cotton or with 5% elastane. Undershirts of the 19th century and early 20th century, as a rule, had 2-3 buttons at the collar. This was justified by the fact that the fabrics were not elastic and the fastener helped not to stretch them.