Square rig

Square rig is a generic type of sail and rigging arrangement in which a sailing vessel uses square sails which are carried on horizontal spars that, when at rest, are perpendicular (or square) to the centre line of the vessel. This is in contrast to fore and aft rig, where the sails are on the vessel's centre line when at rest. A consequence of this is that square sails have a front and a back surface, with the back always facing the wind when used to drive the ship forward, whilst fore and aft sails have a left and right surface, either of which may face the wind. The majority of square sails are symmetrical about an axis parallel to the mast, whilst most fore and aft sails are asymmetrical in shape.

A mast is considered to be fully square-rigged if it has square sails along its full height, including the lower mast. A ship is usually thought of as a square rigger if it has one or more fully square-rigged masts, though British Merchant Shipping Acts have historically had variations on that when deciding on the qualifications needed for some rigs.

Square rig has existed since, at the latest, Pharaonic Egypt, with different versions throughout history. Ships of much of the classical period had square sails with distinctive lead or bone rings which took brailing lines which were used to both furl and reef the sail. This meant that most of the sail handling was done from the deck. Larger classical period ships had more than one mast, sometimes with more than one sail per mast.