square rig refers to a type of sail and rigging where the sails are mounted on horizontal spars. the number of masts, the square sails and how the yards (spars) are set out distinguishes a square-rigged ship. the classic example of a square-rig is a three masted fully rigged ship. the square-rigger descends from the lateen-rig. its angled sails became square sails. the full-rigged ship is a ship with square-rigged sails on all its masts. the square rig with all its sails and rigging is hard to handle when tacking or "coming about". in the trade winds, square-rigged ships have an advantage, especially in deep water.