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A mainsail is the most important sail raised from the main (or only) 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 lowest and largest and often the only sail rigged aft of the main mast, and is controlled along its foot by a spar known as the boom. A sail rigged in this position without a boom is generally called a trysail, and is used in extremely heavy weather.
The modern Bermuda rig uses a triangular mainsail as the only sail aft of the mast, closely coordinated with a jib for sailing upwind. A large overlapping jib or genoa is often larger than the mainsail. In downwind conditions (with the wind behind the boat) a spinnaker replaces the jib.
A roll mainsail is furled by being rolled within (or around) the mast or boom.
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