Leech

The aft edge of a fore-and-aft sail.

This edge runs between the head and clew on many triangular sails and has a strong influence on sail shape and airflow. If it is too tight, the sail may hook and disturb flow; if too loose, it may flutter and wear. Sail controls such as sheet tension, kicker, leech line and traveller position can affect it. Watching this part of the sail helps with trimming for speed, balance and reduced wear.