A boom vang is an item of rigging in a sail-powered vessel (usually small ones, but it is sometimes found on larger ones as well). It runs from the base of the mast to a point mid-way down the boom. Its purpose is to prevent the boom from lifting when the sheet is slackened (either when running "off the wind", or when performing a gybe), and constrain the boom to travel in an arc whose plane is perpendicular to the mast. This article is about the rigging of ships, and is based on the detailed article in the 1911 Encyclopedia Britannica, now in the public domain. ... A sail is a surface intended to generate thrust by being placed in a wind; basically it is a vertically oriented wing. ... mizzen mast, mainmast and foremast Grand Turk The mast of a sailing ship is a tall vertical pole which supports the sails. ... The boom is a spar usually made of aluminum or wood which provides support to the foot of the mainsail. ... In sailing, a sheet is a line attached to the clew of a sail, and is the main control used in trimming the sail. ... Gybe (also spelled jibe) is when a sailing boat (yacht) turns its stern through the wind, such that the wind changes from one side of the boat to the other. ... In mathematics, a plane is the fundamental two-dimensional object. ... For the Deep Purple album, see Purpendicular. ...