The clew is the lower aft (back) corner of a sailboat’s mainsail, located where the foot (bottom edge) and leech (trailing edge) meet. It serves as a critical attachment point for controlling sail shape, primarily secured to the boom via an outhaul, allowing sailors to manage foot tension, draft, and trim.

Key Characteristics and Functions:
Location: The aftmost lower corner of a triangular mainsail, positioned behind the mast.

Control Mechanism: On a mainsail, the outhaul attaches here to pull the clew toward the end of the boom, controlling the tension on the sail's foot.

Attachment Methods: The clew is attached to the boom using a shackle, a clew slug/car that slides in a boom track, or a Velcro strap.

Reefing: During reefing, the reefing line pulls the new reefed clew cringle down to the boom to support the load of a reduced sail area.

Performance: A properly adjusted clew ensures the sail has the correct shape for various wind conditions, affecting the boat's speed and stability.