Reefing lines are specialized control lines used to reduce the surface area of a mainsail in high winds, improving vessel stability by pulling the sail down to the boom. They pass through reinforced "cringles" (grommets) on the sail’s luff and leech, lowering the sail's center of effort, with modern systems running back to the cockpit for safety.

Key Components and Descriptions
Reefing Lines (General): Specifically used lines, often part of a "Jiffy Reefing" (or slab) system, that secure the new tack (forward) and clew (aft) to the boom.

Aft Reef Line (Clew): Runs from the boom, up through the rear reefing cringle (leech), and back to the boom or a cockpit winch, tensioning the foot of the sail flat.

Forward Reef Line (Tack): Secures the front reef point, often a simple hook (ram’s horn) or a dedicated line to the gooseneck.

Single-Line System: A system where one line controls both the tack and clew simultaneously, simplifying the process.

Dual-Line System: Uses separate lines for the tack and clew, allowing for more precise tension adjustments.