A studding sail, studsail or stunsail (traditionally pronounced stuns'l) is a sail used to increase the sail area of a square rigged vessel or 1950's racing skiffs in light winds.
Diagram showing how a studding sail attaches to a yardarm
It is an extra sail hoisted alongside a square-rigged sail on an extension of its yardarm. It is named by prefixing the word studding to the name of the working sail alongside which it is set.
Studding sails have also been used to increase the sail area of a fore-and-aft spanker, again by extending the upper spar. Such a sail extending the leech of a fore-and-aft sail is known as a ringtail. Ringtail sails were used by Flying 18 foot skiffs in Sydney Harbour in the 1950's to enlarge the downwind sailing capacity in light airs. Sails that extend below the boom to deck level or lower are known as watersails.