In sailing, a boom is a spar (pole), along the foot (bottom edge) of a fore and aft rigged sail, that greatly improves control of the angle and shape of the sail. The primary action of the boom is to keep the foot of the sail flatter when the sail angle is away from the centerline of the boat. The boom also serves as an attachment point for more sophisticated control lines. Because of the improved sail control it is rare to find a non headsail without a boom. In some modern applications, the sail is rolled up into the boom for storage or reefing (shortening sail).