A half-pipe is a structure used in gravity extreme sports such as snowboarding, skateboarding, skiing, freestyle BMX, and skating. The structure is wood, concrete, metal, earth, or snow. It resembles a cross section of a swimming pool, essentially two concave ramps (or quarter-pipes), topped by copings and decks, facing each other across a flat transition, also known as a tranny. Originally half-pipes were half sections of a large diameter pipe. Since the 1980s, half-pipes contain an extended flat bottom between the quarter-pipes; the original style half-pipes are no longer built. Flat ground provides time to regain balance after landing and more time to prepare for the next trick. Half-pipe applications include leisure recreation, skills development, competitive training, amateur and professional competition, demonstrations, and as an adjunct to other types of skills training. A skilled athlete can perform in a half-pipe for an extended period of time by pumping to attain extreme speeds with relatively little effort. Large (high amplitude) half-pipes make possible many of the aerial tricks in BMX, skating and skateboarding.