This is one of the two main styles of rigging a sailing ship. It is so called not because of the shape of the sail, but because of the fact the masts and yards are square to the keel. Make sense?

This is taken directly from Wikipedia: 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 at least partially so rigged is called a square rigger.

The sails are actually a trapezoid in shape on just about every square rigged ship. The spars on this type of ship are fixed into position. The spar on a trireme were not fixed, therefore a trireme is not a true square rigged ship. Square rigging is also the oldest form of mounting a sail on a ship. The fore and aft style came later.

Square rigging may seem to give the ship the best bang for the buck as far as speed, but this is not true. A fully rigged square rigged ship taking full advantage of a wind will top out at 6-8 knots. Plus a fully rigged ship requires a larger crew to set and maintain the sails. Most square rigged ships will have some smaller fore and aft rigged sails to slow speed maneuvering.

The discussion of square rigged ships will continue Monday with more information on masts, sail location, and so on.

The picture shows a typical 18th-19th century squard rigged ship.