A half-pipe is a structure use in gravity extreme sports such as snowboarding, skateboarding, skiing, freestyle BMX and inline skating. The structure is usually wood, although sometimes the surface is made of another material such as concrete, metal, dirt or snow. In appearance, it resembles a cross section of a swimming-pool, and in its most basic form, it consists of two concave ramps (or quarterpipes), topped by copings and decks, facing each other across a transition. Originally half-pipes were simply half sections of a large pipe. Since the 1980s, half-pipes have had extended flat ground (the flat bottom) between the quarterpipes and the original style half-pipes have become deprecated. The flat ground gives the athlete time to regain balance after landing and more time to prepare for the next trick.