Concave, vee, roll, channels: these are all examples of bottom contour. Vee (often used in the tail) allows a board to go rail to rail easier. Roll, or "belly", (mostly used on longer boards) provides forgiveness, but decreases speed. Concave, perhaps the most popular of all bottom contours, provides increased speed through lift-air is introduced into the bottom while the board is lifted up on the two contact surfaces of the concave. There is less friction and drag production (picture a catamaran planing across the water). Channels are a more radical form of concave-where concaves allow water to flow in and out in different directions, channels direct the flow of water and air that is introduced more absolutely.