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, beyond the last stay, are called the yardarms. A ship mainly so rigged is called a square rigger.

The square rig is the aerodynamically most efficient running rig (i.e. sailing tailwind), 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.