mainsail

thumb|300px|A square-rigged vessel A mainsail is a sail rigged on the main mast of a sailing vessel. On a square rigged vessel, it is the lowest and largest sail on the main mast. On a fore-and-aft rigged vessel, it is the sail rigged aft of the main mast. The sail's foot is normally attached to a boom. (In extremely heavy weather, the mainsail may be lowered, and a much smaller trysail hoisted in its place).

Historical fore-and-aft rigs used a four-sided gaff rigged mainsail, sometimes setting a gaff topsail above it.