In Sailing a cunningham is a device used on a sloop rigged sailboat or other sailboat with a similarly shaped mainsail to change the shape of the sail. It consists of a line that is secured at one end usually to the mast or boom below the foot of the mainsail. It is passed through a cringle in the luff of the sail near the foot but above the tack and then led down on the other side to a fitting on the mast or boom or on deck. By hauling or easing the line the tension in the luff can be changed which shifts the point of maximum draft of the sail forward or aft optimizing sail shape and therefore performance. It's a fine control which is used more frequently on racing sailboats than it is on cruising or day-sailing boats.
It is named after its inventor Briggs S. Cunningham II, racecar enthusiast and victorious America's Cup sailor.
