A leech is the trailing or aft edge of a fore-and-aft sail, extending from the top corner (head) to the rear corner (clew). It is a critical, actively adjusted edge that dictates sail shape, controls aerodynamic twist, and acts as the primary rudder to influence boat speed, usually managed by the mainsheet and vang.

Key Aspects of the Leech:
Location: The rear edge of a mainsail or jib, located behind the mast.

Function: It allows air that is bent around the front of the sail (the luff) to exit, creating lift and forward motion.

Adjustments:
Tension: Tightening the leech (closing it) adds power but can cause stalling; easing it (opening it) allows the top of the sail to twist, reducing power in high winds.

Battens: Stiffening elements inserted along the leech to prevent it from flapping and to maintain a curved, efficient shape (roach).

Telltales: Small ribbons attached to the leech to show the airflow behavior and indicate when the sail is stalling.