Shape & trim
Twist
The sail's upper part opening further than its lower part — usually a good thing.
DutchTwistour Dutch loft →
SpanishTorsiónour Spanish loft →
Twist is the difference in angle between the bottom of the sail and the top. Sight up the leech and you will see the head is trimmed further out than the foot: that is twist, and it is deliberate.
It exists because the wind up your mast is not the wind at deck level. Friction slows air near the water, so the apparent wind aloft is both stronger and further aft. A sail with twist meets the wind at the right angle all the way up, instead of being right at one height and wrong everywhere else.
You control it with the sheet, the traveller and the boom vang. Too little twist — a leech strapped tight — stalls the top of the sail and heels the boat. Too much, and the head is just flapping.