Sail (derived from Latin sagulum, "a cloak "), a device used on board a boat or ship for catching the wind and so propelling the vessel. It general1y consists of several breadths of canvas, served with a double seam at the corners, and edged by cords called bolt-ropes. Sails used on square-rigged vessels and fixed on yards are called square-sails; those fixed on a gaff, boom, or stay, are called fore- and- aft sails. The top of a square sail is the head, the bottom the foot, the weather- or windward-side is called the luff, the other side the after-leech. The two lower corners are called clues, the weather clue being the tack. The sails take their prefix from the masts, and consist of courses, topsails, and topgallant-sails. Other varieties are lug-sails, which are extended on a yard hauled nearly to the top of a mast, spritsails, the outer upper corner of which is extended by a sprit or boom going from the bottom of the mast, and lateen sails, which are much used in the East and have a long yard or boom affixed to a short mast. Many other sails are also in use, and on some yachts silk is employed as a materia1. Sails are also used on windmills to catch the wind.