Studding sail

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 1950s racing skiffs in light winds.

It is an extra sail hoisted alongside a square-rigged sail on an extension of its yardarm. It is named by appending the word studding to the name of the working sail alongside which it is set. These sails provide extra speed in fine weather.