Parts of a sail
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In theory, a sail can be thought of as a triangle, with three sides (edges), and three corners.
The foot of a sail is its bottom edge, which, on a mainsail, runs parallel to the boom. The forward (leading) edge of the sail is called the luff, which, in a mainsail, is parallel to the mast. The aft (back) edge of a sail is called the leech (also spelled leach), which is opposite the luff, and forms the hypotenuse of the triangle
In a triangular sail, the corner where the luff and the leech connect is called the head.  The corner where the luff and foot connect is called the tack., and, on a mainsail, is located where the boom and mast connect. The corner where the leech and the foot connect is called the clew. In the case of a symmetrical spinnaker, each of the lower corners of the sail is a clew. However, when used the corner to which the spinnaker sheet is currently attached is called the clew, and the corner attached to the spinnaker pole is referred to as the tack.
The shape of a sail is seldom a perfect triangle. It is common for sailmakers to add an arc of extra material, often on the leech, outside of a direct line drawn from one corner of the sail to the other along that edge. This additional part of the sail is known as the roach; which is commonly found on most modern racing sails. The roach adds considerable additional sail area, giving the sail more “power” than it otherwise would have. Battens are often necessary to stabilize the leech of the sail, especially if there is any pronounced roach. Without battens, the roach would be damaged, possibly destroyed, by wind-induced flogging.
Quadrilateral fore-and-aft sails
Each of the corners on a quadrilateral fore-and-aft rigged sails has its own name:
The throat is the upper forward corner of the sail.
The peak is the upper aft corner. Gaff rigged sails, and certain similar rigs, employ two halyards to raise the sails: the throat halyard raises the forward, throat end of the gaff, while the peak halyard raises the aft, peak end.
The tack is the lower forward corner of the sail.
The clew is the lower aft corner.
The four edges of a sail are likewise independently named.
The foot, as in a triangular sail, is the bottom edge of the sail, which runs roughly parallel to the deck. The foot is often attached, at the tack and clew, to a boom; if no boom is present, the sail is said to be "loose-footed."
The head is the upper edge of the sail, and is attached at the throat and peak to a gaff, yard, or sprit.
The luff is the forwardmost vertical edge of the sail, which runs along the mast.
The leech is the aft vertical edge of the sail.
Many of the same names are used for parts of a quadrilateral square rigged sail.
- As for a triangular sail, this refers to the topmost part. On a square sail, however, this part is an edge rather than a corner.
- The "side" edge of the sail. Since square sails are symmetrical, they have two leeches. Occasionally, when the ship is close-hauled, the windward edge of the sail might be referred to as the luff.
- Like a triangular sail, the "free" corners of a square sail are called clews; again there are two of them. Square sails have sheets attached to their clews like triangular sails, but the sheets are used to pull the sail down to the yard below rather than to adjust the angle it makes with the wind.
- The bottom edge of the sail.
'Clew lines' are ropes attached to the clews, and 'clewgarnets' or 'cluegarnets' are the tackles attached to clew lines. These lines and tackles are used to ‘clew up’ the ‘courses’ of a square sail (i.e. to pull the clews up onto the upper yard or the mast in preparation for furling the sail).
'Buntlines' are ropes attached along the foot of a square sail and led through 'lizards' up the front of the sail to assist with clewing up the sail.
'Slablines' are the equivalent of buntlines but run up the back of the sail.
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