What is Square rig?

Square rig is a generic type of sail and rigging arrangement in which the primary driving sails are carried on horizontal spars which are perpendicular, or square, to the keel of the vessel and to the masts. These spars are called yards and their tips, outside the lifts, are called the yardarms. A ship mainly rigged so is called a square-rigger.

The square rig is aerodynamically the most efficient running rig , and stayed popular on ocean-going sailing ships until the end of the Age of Sail. The last commercial sailing ships, windjammers, were usually square-rigged four-masted barques.