Size-free underwear generally refers to a product introduced by Ubras, which subverts the traditional definition of underwear size, adopts super-elastic fabrics and original patented design, and uses one underwear to meet the needs of different bodies, thus lowering the selection threshold for users to buy underwear. Because it is comfortable to wear, it is especially suitable for long-distance flights or trains, so it is called "airplane bra". Underwear refers to clothes attached to the body, including Chinese-style chest covering, undershirt, sweat bag, half arm, tube top, bra and so on. , usually in direct contact with the skin, is one of the indispensable clothes for modern people.