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. 