Mainsail

A mainsail is a sail located behind 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 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.