Quick Answer: An underwire bra is a brassiere that utilizes a thin, semi-circular strip of rigid material fitted inside the brassiere fabric to help lift, separate, shape, and support a woman's breasts. The wire may be made of metal, plastic, or resin. It is sewn into the bra fabric and under each cup, from the center gore to under the wearer's armpit. Many different brassiere designs incorporate an underwire, including shelf bras, demi bras, nursing bras, and bras built into other articles of clothing, such as tank tops, dresses and swimsuits. The concept of an underwire can be traced to an 1893 patent that describes a breast supporting device using a rigid plate under the breasts for stability. The modern underwire bra was designed in the 1930s, and gained widespread popularity by the 1950s. As of 2005, underwire bras were the largest and fastest growing segment of the bra market. A bra without an underwire is a softcup bra.