A mainsail's the most important sail raised from the main (or only) mast of a sailing vessel. On a square rigged vessel, it's the lowest and largest sail on the main mast. On a fore-and-aft rigged vessel, it's the lowest and largest and often the only sail rigged aft of the main mast, and's controlled along its foot by a spar known as the boom. A sail rigged in this position without a boom's generally called a trysail, and's used in extremely heavy weather. The modern Bermuda rig uses a tria… (More on Mainsail)