Topsail

A topsail ("tops'l") is a sail set above another sail; on square-rigged vessels further sails may be set above topsails.
Square rig.
On a square rigged vessel, a topsail is a typically trapezoidal shaped sail rigged above the course sail and below the topgallant sail where carried ,on any mast (i.e., a fully rigged ship would have a foremast topsail, a mainmast topsail, and a mizzen topsail). A full rigged ship will have either single or double (i.e, "split" upper and lower) topsails on all masts, the single or lower topsail being the second sail above the deck and the upper topsail where so rigged being the third.
Although described as a "square" sail, a topsail on a full rigged ship refers not to the sail's shape but to it and its yard being rigged square (i.e., at a right angle) to the vessel's keel rather than in line with it (in which case it would be called a fore-and-aft rig or a fore-and-aft rigged sail) ; a square rigged topsail is nearly always trapezoidal in shape, with the lengths of the upper yards being progressively smaller the higher they are on the mast. The bottom edge of the topsail, like that of other square rigged sails, is slightly concave which allows the rigging to connect to the mast. The principal exception to this trapezoidal rule is the raffee sail, which is a square rigged topsail that is triangular (such as is seen on the foremast of the ship "Denis Sullivan", a topsail schooner). A topsail which is fore-and-aft rigged is usually also triangular, but has its longest edge oriented vertically rather than horizontally as is seen in the raffee sail.
Although the early Romans used a sort of fore-and-aft rigged topsail on some vessels, this sail first came into prominent use in Europe some time in the 15th century. Initially small and carried only on main and fore masts, they gradually increased in size and importance until by the middle of the 17th century and were the principal and largest sails of the ship, the first sails to be set and the last to be taken in. It was quite common for a ship to sail with topsails and jibs alone; the position of the topsails well above the sea ensured that they received a steady breeze even if the seas were rough.
Because of their ability to drive a ship in lighter winds than the course sail below or any sail above, the topsail quickly became enormous and was soon difficult and dangerous to handle in strong winds. Sometime in the 1680s, reef-bands were introduced to tie up part of the sail, with topsails eventually getting four of these. In the mid 19th century topsails of merchant vessels were split into separate upper and lower topsails that could be managed independently and far more easily by significantly smaller crews (see "Cutty Sark" for a prominent example). Such sails were still often referred to as a single "topsail", however.
Competing versions of this double topsail were invented by Robert Bennet Forbes and Captain Frederic Howes. Although Forbes strove to defend his rig, the Howe rig dominated. In the Forbes rig, both topsail yards are fixed vertically. In the Howe rig, the upper topsail yard slides on the topmast so it can be lowered in a few seconds to close reef the upper topsail. Howe had the foot of the upper topsail closely attached to the lower topsail yard. In 1865 the British clipper "Ariel" introduced a gap there. Forbes first tried his rig in the topsail schooner "Midas" in 1844.The clipper "Climax" built in 1853 under the supervision of Howes was the first ship with Howe rig.
Gaff rig.
Gaff topsails, like gaff rigs in general, may still be seen at tall ships gatherings. The gaff rig has been largely superseded by the Bermuda rig, which has no topsails. On a gaff-rigged sailing boat, topsails may take a few different forms:
Spritsail rig.
The use of the topsail in Thames sailing barges is significantly different. It is a fore and aft rig, where the mainsail is controlled by brails. In confined waters, the barge will sail under topsail and mizzen, taking advantage of the clear air above the lee of the buildings. The topsail was attached by hoops to the topmast, so could be rapidly struck, the release of one rope let it fall.
The topsail a fore and aft sail.
The jib topsail a staysail set between the topmast and the bowsprit.
Other uses.
On rigs having multiple jibs or staysails of which at least one is set high, such as many late 19th and 20th Century racing cutters, the uppermost of these, set flying or on a topmast stay, is often called the jib topsail.
Roman navigation.
Topsails (Greek: "sipharos"; Latin: "siparum") in the form of an isosceles triangle set above the square mainsail were used in Roman navigation.