The main sail is an important part of a ship's rigging. Also known as the mainsail, it is the largest, most prominent sail on a ship and is usually positioned on a mast that runs vertically up through the middle of the vessel. Other terms often used synonymously for the main sail include the primary sail, the fore-and-aft sail, or the course sail. It is the sailing vessel's primary source of wind power and is responsible for propelling the ship forward. Other sails may be used in conjunction with the main sail, but it remains the central sail that determines the vessel's overall course and direction.