Shape & trim
Luff round
The outward curve cut into the luff, which becomes belly when the sail goes up the mast.
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A mainsail's luff is not cut straight. It is cut with an outward bow — the luff round — and when you hoist it, the mast is straight (or straighter) and that surplus cloth has to go somewhere. It goes into the sail as depth. Flat panels, curved wing.
The size of the curve is matched to your mast's bend, which is why the same sail on a bendier rig comes out flatter. Tension the backstay, the mast bends forward in the middle, it eats more of the round, and the sail flattens. That is not a side effect — it is the design working.
Get it wrong in either direction and no amount of trimming fixes it. It is one of several reasons we confirm the real rig before cutting rather than trusting a table.