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File:Yacht mainsail.svg
The mainsail of this cutter is indicated in red

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.

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

A roll mainsail is furled by being rolled within (or around) the mast or boom.

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