Square rig is a generic type of sailing vessel in which the main horizontal spars are perpendicular to the keel of the ship. These spars are called yardarms or simply yards. Square rig was the main design in the age of sail.

Square rig can also describe individual four-cornered sails suspended from yardarms, and carried on either a square rigged or a mainly fore-and-aft rigged vessel, such as one with either a bermuda rigged or gaff rigged mainsail.

A mast is considered square rigged if its course is square rigged, but normally if this is the case it will have a complete set of square rigged sails. If the course is fore-and-aft, square topsails can still be carried in front of the mast.

Square rigged vessels range in size from small sloops to full rigged ships. However even for the largest vessels, the rig has been largely replaced by fore-and-aft gaff rigs and bermuda rigs, which give superior all-around performance with far smaller and less skilled crews. Similarly, hybrid vessels carrying some square-rigged sails have largely disappeared.

A square rigged sail is not in fact square, but more nearly trapezoidal, being symmetrical but longer in the foot than the head. Like all sails it is three-dimensional, and its curve or belly means its foot is not a straight line at all. It is fixed to a spar along its head, and its clews are controlled by sheets, often run to blocks on the spar immediately below the sail.

Modern square rigged ships are still used for training, tourism and ceremonial purposes.

Types of square rig

Principally square rigged types

• A barque has three or more masts, with the aftermost entirely fore-and-aft rigged, while the fore, main and any others are square rigged.
• A brig has two masts, both square rigged.
• A full rigged ship has three or more masts, including a foremast, mainmast and mizzen, and all masts are square rigged.
• A sloop has only one mast.

All the above rigs normally carry a number of jibs and at least one spanker, and may also carry fore-and-aft staysails between the masts.

Fore-and-aft rigs with some square rigged sails

• A barquentine has three or more masts, the fore square rigged, the main, mizzen and any others fore-and-aft rigged.
• A brigantine has two masts: A square rigged foremast and a fore-and-aft rigged mainmast.
• A square-topsail ketch carries one or more square-rigged topsails (commonly two, an upper and a lower topsail) on its mainmast, in addition to its mainsail, mizzen sail and jibs.
• A square-topsail schooner carries one or more square-rigged topsails on its foremast. This rig is often simply called a topsail schooner as topsails above the gaff-rigged sails are a standard part of the schooner rig.
• A square-topsail cutter or sloop carries one or (less commonly) two square topsails on its only mast, in addition to its mainsail and jib or jibs.

Ranks and Duties of Officers and Sailors

Fully-rigged sailing vessels required a phenomenal expertise to get them under weigh and out to sea. One of the best scholarly articles on the subject is Organization and Incentives in the Age of Sail (http://business.clemson.edu/Spiro/images/pdf/WP01-102.pdf), which lists over fifty occupations and positions held by British sailors of the era.