Parts of a sail
Leech
The trailing edge, from head to clew - where the wind leaves the sail.
DutchAchterlijk
SpanishBalumaour Spanish loft →
The leech is the after edge of the sail. It is where air exits, so it does more than any other edge to decide how the sail behaves: too tight and the sail stalls and heels the boat, too open and you spill power.
On a mainsail the leech is usually curved outward well past the straight head-to-clew line. That extra area is the roach, and it is held out by battens. On a genoa the leech is normally cut slightly hollow instead, so it doesn't hook into the mainsail.
Most sails carry a thin adjustable line inside the leech tape to stop it fluttering. Flutter is not cosmetic - it is the single fastest way to destroy a sail. See sail care.