Reefing is the means of reducing the area of a sail, usually by folding or rolling one edge of the canvas in on itself. The converse operation, removing the reef, is called "shaking it out."  Reefing improves the performance of sailing vessels in strong winds, and is the primary safety precaution in rough weather. Reefing sails improves vessel stability and minimizes the risk of damage to the sail or other gear. Proper skills in and equipment for reefing are crucial to averting the dangers of capsizing or broaching in heavy weather.
Sails may have built-in alternative attachment points that allow their area to be reduced. In a mainsail, pairs of grommets, calling reefing tacks and reefing clews, may be installed in the sail; a cruising boat will typically have two to three pairs. Pulling these points down to the boom forms a new tack and clew, reducing the sail's area. Using the pair of grommets closest to the boom is called a single reef, using the next pair is called a double reef, and so on. A sail may have reef points, small holes in the sail between the grommets. These are used to secure the excess fabric of the sail after reefing to minimize flogging and improve visibility from the cockpit.  To reef the sail of a Bermuda rigged sloop:
- The boat should be brought head-to-wind (in irons).
- Lower the jib and main sail. Leaving the jib up allows it to turn the boat away from the wind. Lowering the main just enough to reef runs the risk of having the wind change directions, blowing the main outboard and filling the sail. Set the boom in the boom crutch and pull the sheet tight.
- Lash the tack reef cringle to the boom, or secure on reefing hook if present.Pull the leech reef cringle away from the mast, stretching the sail taut along the boom. Lash the leech cringle to the boom, and then to the end of the boom. Then tie the remaining reefs with sail ties. With lighter gauge sails or reef points reef the sail to itself rather than around the boom to prevent the reefing cringles from being torn out. Secure the sail ties with a reef (square) knot.
- Once the sail is reefed, raise the main, stow the boom crutch, raise the jib, and bear off. If the reefing is done while moored or in sheltered waters, the sail to be reefed need not be lowered entirely. One crewman must pull the reefing line as another crewman lowers the sail.
Roller reefing rolls or wraps the sail around a wire, foil, or spar to reduce its exposure to the wind. In mainsail furling systems the sail is either wrapped around the boom by a mechanism in the gooseneck or hardware inside the boom winds it around a rotating foil.
Furling systems controlled with lines led to the cockpit allow reefing without crew having to go on deck in heavy weather. Roller reefing also allows more variable sail area than conventional or jiffy reefing. Countering these advantages are the furled sail possibly not having an optimal shape and sail repair or replacement being more difficult.
Jiffy reefing, also called slab or single line reefing, is an alternative to conventional or roller reefing where the sail is folded in sections, or slabs, along the boom. One or two reefing lines reeved through the sail's luff and leach reef cringles create a new tack and clew for the sail by pulling those points tight to the boom. These can be led back to the cockpit to allow crew members to reef without going on deck in heavy weather. Intermediate reef cringles are not used.
- Bingham, Bruce (20 May 2015). "How to Reef the Main". Cruising World. Retrieved 3 April 2016.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
- Jobson, Gary (1998). Sailing Fundamentals. Simon & Schuster. ISBN 0684849941.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
- Gauther, Stan. "Reefing Preferences from the Pros". Practical Sailor. Retrieved 27 March 2016.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>