A mainsail is the largest and most important sail on a sailing ship or boat. It's called the “main” sail because it does the main work of catching wind and moving the vessel forward.

On most boats, the mainsail attaches to the tallest mast (the vertical pole rising from the deck) and stretches out along a horizontal beam called the boom. When wind fills the mainsail, it billows out like a giant curved wing, pulling the boat through the water. Sailors adjust the mainsail constantly, tightening or loosening ropes to catch the wind at just the right angle.

Different types of sailing vessels position their mainsails differently. On a traditional sailboat with triangular sails, the mainsail sits behind the mast. On old square-rigged ships like those used by explorers and navies centuries ago, the mainsail was a large rectangular sail hanging from the mainmast.

Learning to handle the mainsail is one of the first skills new sailors master. Raising it means you're ready to harness the wind's power. Lowering it means you're ready to slow down, stop, or switch to engine power. Without a properly trimmed mainsail, even the most beautiful boat becomes difficult to control.