Square rig is a generic type of sail and rigging arrangement

Sail-plan

A sail-plan is a set of drawings, usually prepared by a naval architect. It shows the various combinations of sail proposed for a sailing ship.The combinations shown in a sail-plan almost always include three configurations:...

 in which the primary driving sails are carried on horizontal spar

Spar

In sailing, a spar is a pole of wood, metal or lightweight materials such as carbon fiber used on a sailing vessel. Spars of all types In sailing, a spar is a pole of wood, metal or lightweight materials such as carbon fiber used on a sailing vessel. Spars of all types In sailing, a spar is a...

s which are perpendicular, or square, to the keel

Keel

In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...

 of the vessel and to the masts. These spars are called yards

Yard (sailing)

A yard is a spar on a mast from which sails are set. It may be constructed of timber, steel, or from more modern materials, like aluminium or carbon fibre. Although some types of fore and aft rigs have yards , the term is usually used to describe the horizontal spars used with square sails...

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.