A term that refers essentially to the use of square-cornered sails, usually approximately rectangular, set from horizontal spars, or yards, balanced across the mast. This arrangement is used from simple single sails to the complex and sophisticated multiple sail rigs developed over 2,000 years for the major ships of war, commerce, and exploration.
Vessels setting square sails are known from the earliest depictions of ships up to the present. The earliest sails may have been animal skins or woven fabrics, both essentially rectangular, which may explain the origins and early dominance of the form. A square sail hung from a spar balanced across a mast has the characteristics of maximizing the sail area for the height of mast while minimizing the operational loads to be handled by the crew. This favoured its use for light open craft where the mast might have had to be set up by those on board when sail was required and also for heavy vessels which needed sail area for power to drive them.
An early use of the square sail is evidenced by the contemporary depictions of Greek galleys some 3,000 years ago. For such craft the limitations of the light hull meant that the square sail was essentially used for running free and reaching, a pattern of use that persisted up to perhaps the 10th century ad. The single square sail was generally set from the deck by hoisting it on a yard and it was removed by lowering the yard to the deck. To meet stronger wind conditions the sail could be replaced by a smaller version. A later practice was to add area to a small and strong sail by lacing one or more lighter additions, called bonnets, along the foot of the sail. These had the value of being able to be added or removed without slowing or stopping.
On early and small vessels the sail was set on a mast with simple supporting rigging and the sail loads taken largely by braces running aft from the ends of the yard and by sheets attached to the lower corners. The yard could be swivelled around the mast under the control of, and limited by, the braces. The leeward-side rigging of the mast itself could be disengaged to allow closer bracing. The windward working value of this manoeuvre was mostly limited by the hydrofoil abilities of the shallow hulls.
As ships grew bigger and heavier to meet more ambitious voyages, two factors controlled their planning. First was the need to increase sail area and this was done by multiplying the number of masts and adding further masts above these masts, each carrying its own sail. The other factor was to keep the length of the ship short, first because of the practicalities of the use of one-piece wooden keels, and second to maximize the relative water flow around the hull which had come to be recognized as an essential component in windward ability.
A typical ship of the 15th–16th centuries might, for instance, carry three masts, square rigged on the forward two and with a bowsprit carrying a square spritsail. The third mast would probably carry a lateen steering sail. Some would carry a second tier of square sails called topsails and in a few years these would extend to a third tier of topgallant square sails.