Naming of Parts - The Boat from the Side
Here are the principal parts of the boat that it is as well to know, visible from the side. This is a modern racing boat, so the mainsail in particular looks quite an unusual shape, very square at the top, but the same names apply.
Parts of the Sails
Racing Dinghy sails are vaguely triangular. Each has three corners and three edges, and each has a different and special name, Suprisingly perhaps the names are the same for each type of sail. The top corner is the head, the front corner the tack and the back corner is the clew. The leading edge is the luff, the back edge the leach, and the bottom edge the foot. When it comes to conventional (symmettrical) spinnakers, where the front and the back edges change around when you gybe, only the luff and tack tend to be referred to. You'll find the word tack (of course) and luff are also used for manouvers. My guess is that this dates back to square rigger days, when those parts of the sail were most affected by those manouvers.
Apropos of nothing, if a sail disintegrates because of the pressure of the wind (which only really happens in big offshore racing boats) it referred to as being blown out. This leads me on to possibly the oldest and corniest joke in sailboat racing, which I heard very many years ago...
Q. What did the jib say when it got blown out?
A. I haven't got a clew...