Reefing

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Reefing reduces the area of a sail, usually by folding or rolling one edge of the canvas in on itself and attaching the unused portion to a spar or a stay, as the primary measure to preserve a sailing vessel's stability in strong winds. Restoring full sail area is termed shaking out a reef.^[1]

Whereas fore-and-aft rigged vessels store the unused portion of the sail on a boom (below the sail), square-rigged vessels stow the unused portion on a spar above the sail. Reefing may occur by rolling the sail around its luff or foot, either on a rotating stay or within a spar.