On a sailing vessel, a topsail is a sail which is set above a lower sail. The exact position and function of a topsail varies depending on the type of rig used by the vessel. On square-rigged vessels, topsails are the most commonly used type of sail.
Square-rigged sailing vessels are so called not because their sails are square — they are trapezoidal, being narrower toward the top — but because the spars or yards which hold up the sails are set at right angles to the masts. The lowest sails in a square-rigged vessel are called the courses or course sails. The topsails are above the courses. In some ships, the two sails above the course are identified as the upper and lower topsails.
Despite their name, topsails are not the topmost sails in square-rigged sailing vessels. Above them may be topgalant or topgallant sails, which are also sometimes divided into upper and lower. Above those come the royal, the skysail, and the moonsail or moonraker. It would be very rare for a square-rigged vessel to set all of these sails at the same time, though.
Initially, the topsail was only a small sail set above the course, which was the main propulsion of the ship. However, from the 16th century onward, it became larger and more important. In the 18th century, it was the most commonly set sail. Its height above the water meant that it could still catch a breeze when rough seas were preventing the courses from receiving wind.
In a gaff-rigged sailing vessel, the arrangement of sails is very different from that of a square rig. A gaff rig is a fore-and-aft rig where the spar, or gaff, is at an angle to the mast rather than straight across it as in a square rig. Some sails on square-rigged vessels are rigged fore-and-aft, but vessels are only said to be gaff-rigged if their mainsails are rigged in this manner. Gaff rigs are no longer common, having been largely superseded by a similar rig called a Bermuda rig.
In gaff-rigged vessels, the topsails are triangular and much smaller than in square-rigged ones. They are set between the tops of the masts and gaffs. In some cases, a small yard, called a jack-yard, may be set, extending the lower edge of the topsail past the gaff. In some gaff-rigged vessels, topsails may even be set square-rigged.