Square sails are a type of sail that is shaped like a rectangle and is typically rigged on a horizontal spar called a yard. They are one of the oldest sail designs, used by ancient civilizations such as the Phoenicians and Vikings. Square sails are effective for sailing downwind, as they catch the wind directly, allowing for good speed and maneuverability. These sails are often associated with traditional sailing ships, including galleons and tall ships. While they provide excellent propulsion when the wind is favorable, they can be less efficient when sailing against the wind. Modern sailing vessels often use a combination of square sails and other sail types for improved versatility.