On a sailing vessel, a staysail is a fore-and-aft rigged sail set on lines which run diagonally downward from a mast. These lines help support the weight of the mast and are called stays. Sails attached to them are therefore called staysails. A single large vessel can have a large number of staysails, although it would be rare for all of them to be set at once. If the sail is forward of the foremast, it can also be called a foresail or headsail.
On a square-rigged vessel, the staysails are usually the only fore-and-aft rigged sails. Square-rigged sails are set at right angles to the line of the vessel, while fore-and-aft sails get their name from their position on the stays which run forward and aft from the masts along the line of the keel. The larger sails on a square-rigged vessel are usually four-sided sails, such as the courses and topsails. The largest and most prominent of the fore-and-aft sails is the jib.
In fore-and-aft rigged vessels, especially smaller vessels such as yachts and sloops, the staysail is a very important part of the rig and may be among the vessel's larger sails. In the cutter rig, for instance, two or more large staysails are set between the single mast and the bowsprit. The innermost of these is simply called the staysail while the outermost is the jib.
The jib, found in both square-rigged and fore-and-aft vessels, is a staysail set forward of the foremast. In a vessel with a bowsprit, the stay on which the jib is set runs between the foremast and the bowsprit, while in others it runs between the foremast and the bow or deck. A vessel can have several jibs, each set to a different point along the bowsprit. In square-rigged vessels, four jibs are common. These are called, from outward to inward, the flying jib, the outer jib, the inner jib, and the foretopmast staysail.
A number of variations exist in types of staysail, both in shape and in how the sail is set. For instance, in the case of the genoa staysail, often simply called the genoa, the clew, or bottom aft corner of the sail, is located aft of the mast. The fisherman's staysail is a rare four-sided example. This large sail is set very high between the fore and mainmasts of vessels such as schooners and brigantines to catch lighter breezes.