A leech line is a small-diameter control line housed within a pocket along the trailing edge (leech) of a sail, running from the head to the clew. It is used to adjust tension to prevent fluttering, improve aerodynamic shape, and protect the sail fabric from damage caused by excessive flexing in higher winds.

Key Aspects of Leech Lines:
Function: The primary purpose is to eliminate "leech flutter" (or flogging). By tightening the line, the trailing edge is tensioned to stop the flutter, which can otherwise cause structural fatigue in sailcloth.

Adjustment: The line typically exits the leech near the clew and is secured in a small cleat. It should be tightened as wind speed increases and released in light air to allow the sail to open up.

Location: While standard lines run from top to bottom, some, particularly for high-boom boats, use an over-the-head setup running from the clew up to the head and down the luff.

Components: It consists of the line itself, a pocket, and a adjustment cleat, often found on both mainsails and genoas.

Other Benefits: Proper tensioning of the leech line helps maintain the designed draft of the sail, contributing to better aerodynamic efficiency.