A sailboat backstay is a crucial piece of standing rigging extending from the top of the mast to the stern, providing structural support to prevent the mast from falling forward. It counteracts the forward pull of the forestay, directly influencing mast bend, headstay sag, and mainsail shape.

Key Features and Functions:
Support: Protects the mast from collapsing forward, particularly in heavy air.

Sail Control: Tightening the backstay increases tension on the forestay (reducing jib sag) and bends the mast, which flattens the mainsail to depower it in higher winds.

Types:
Permanent Backstay: Attached directly to the masthead, often with a mechanical (screw) or hydraulic adjuster.

Split Backstay: The wire splits into two "legs" near the deck to fasten on both sides of the transom, providing clearance for the helm.

Running Backstays: Removable or adjustable lines used on fractional rigs to support the mast below the masthead.

Materials: Typically constructed from stainless steel wire, rod rigging, or modern high-strength synthetic fiber.