The term leechline is used to mean two different things:
- A thin line running through the leech of a sail that can be tensioned or slacked. This line can be tightened to prevent the the leech of the sail from flapping in the wind. If this line is too tight, the leech of the sail will become "cupped" and will prevent air flow from smoothly leaving the leech of the sail. If a sail has an adjustable leech line, there is usually a small cleat sewn into the clew of the sail, and perhaps other small cleats near the reef points.
- A small piece of yarn or a thin strip of material sewn into the leech of a sail. A mainsail usually has several of these lines along its leech to allow a sailor to visualize airflow. If the leechlines are streaming backwards, then air is flowing smoothly over the leech of the sail at that point. If they are spinning or pointing backwards, then the airflow is stalled or turbulent. This lines are often also called telltales.