A studding sail or studsail is a sail

Sail

A sail is any type of surface intended to move a vessel, vehicle or rotor by being placed in a wind—in essence a propulsion wing. Sails are used in sailing.-History of sails:...

 used to increase the sail area of a square rig

Square rig

Square rig is a generic type of sail and rigging arrangement in which the primary driving sails are carried on horizontal spars which are perpendicular, or square, to the keel of the vessel and to the masts. These spars are called yards and their tips, beyond the last stay, are called the yardarms...

ged vessel in light winds. Traditionally pronounced stuns'l.

It is an extra

Extra (sailing)

In sailing, an extra is a sail that is not part of the working sail plan.The most common extra is the spinnaker. Other extras include studding sails, the modern spanker , and some staysails and topsails....

 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

Spanker (sail)

A spanker is either of two kinds of sail.On a square rigged ship, the spanker is a gaff rigged fore-and-aft sail set from and aft of the aftmost mast. Almost all square rigs with more than one mast have one or two spankers, which evolved from the driver sail. Some also carry a topsail above the...

, again by extending the upper spar

Spar

In sailing, a spar is a pole of wood, metal or lightweight materials such as carbon fiber used on a sailing vessel. Spars of all types In sailing, a spar is a pole of wood, metal or lightweight materials such as carbon fiber used on a sailing vessel. Spars of all types In sailing, a spar is a...

. Such a sail extending the leech of a fore-and-aft sail is known as a ringtail. Sails that extend below the boom to deck level or lower are known as watersails.