The clew is the lower aft (back) corner of a triangular sail, where the leech (trailing edge) and foot (bottom edge) meet. It is a primary control point for adjusting sail shape and tension via sheets, outhauls, or straps to optimize performance. On a mainsail, it attaches to the boom; on headsails, it is used for sheeting.

Key Aspects of the Clew:
Function: Controls the tension and angle of the sail, affecting its shape (flatness or fullness).

Mainsail Attachment: Connected to the boom using an outhaul or a clew strap to manage foot tension.

Jib/Genoa Attachment: Attached to sheets, which are used to trim the sail's angle relative to the wind.

Sailing Application: Tightening the clew (via outhaul) flattens the sail for heavy wind, while loosening it increases draft for light wind.

Symmetrical Spinnakers: In these, both lower corners are considered clews, with the sheet attached to the leeward corner.

The clew is critical for trimming a boat to sail faster, tighter, or in changing conditions, particularly by managing leech tension and controlling the sail’s depth.