The leech is the trailing or back edge of a sail, extending from the head (top corner) to the clew (lower-back corner). It controls the sail's aerodynamic shape, twist, and efficiency. Proper leech tension, often managed with a leech line to prevent flapping, is crucial for preventing drag and maintaining performance.

Key Aspects of the Leech:
Position: It is the aft (rear) edge of both mainsails and jibs.

Shape: Often has a curved, protruding edge called a "roach" that needs to be supported, typically by battens.

Leech Line: A small line run along the edge to tighten it, which stops high-wind flutter and prevents damage.

Performance Control: Tension is adjusted via the mainsheet, boom vang, and traveler. Tightening the leech flattens the sail; easing it increases twist and power.

Components: Often contains battens to help maintain the desired airfoil shape.