square dance

Dance for sets of four couples standing in square formation. The most popular type of U.S. folk dance, it derived from the quadrille and was originally called a square dance to distinguish it from the contra, or longways, dance (for a double line of couples) and the round dance (for a circle of couples). The U.S. square dance progresses through specific patterns called or sung out to the dancers by a caller and accompanied by lively music played on instruments such as fiddle, banjo, accordion, guitar, and piano.

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In measurement, a device consisting of two straightedges set at a right angle. It is used by carpenters and machinists to check the correctness of right angles, as a guide when drawing lines on materials before cutting, or for locating holes. In mechanical drawing or drafting, a T-shaped instrument known as a T square is used to establish a horizontal reference on the drafting board.

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Geometrical representation of the traditional logical relations of opposition (contradictories, contraries, subalterns) among the four types of propositions (universal affirmative or A-proposition, universal negative or E, particular affirmative or I, particular negative or O) in the Aristotelian doctrine of the syllogism. At the top left is the A; at the top right is the E; below the A is the I; and below the E is the O. The A and the O, like the E and I, are contradictories (diagonal relations); the A and the E are contraries; and the I is subaltern to (hence implied by) the A, the O subaltern to the E. Whereas contradictories have opposite truth-values (one true, the other false), contraries cannot both be true but can both be false.

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One of the largest public squares in the world, originally designed and built in Beijing in 1651 and enlarged in 1958. It is named for the massive stone “Gate of Heavenly Peace” (Tiananmen) at its northern end. It contains and is surrounded by halls, museums, and monuments, including the Mao Zedong Memorial Hall, where Mao's body rests in state. Tiananmen Square is the site of numerous parades and other celebrations. It has also been the rallying point for political demonstrations, including events during the May Fourth Movement in 1919 and during May–June 1989.

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In American English, a set square is an alternative name for a T-square.

A set square or triangle (American English) is an object used in engineering and technical drawing, with the aim of providing a straightedge at a right angle or other particular planar angle to a baseline.

The most simple form of set square is a triangular piece of transparent plastic (or formerly of polished wood) with the centre removed. More commonly the set square combines this with a ruler and a half circle protractor, like on the picture. The outer edges are typically bevelled. These set squares come in two usual forms, both right triangles: one with 90-45-45 degree angles, the other with 30-60-90 degree angles. They are often purchased in packs with protractors and compasses.

Less commonly found is the adjustable set square. Here, the body of the object is cut in half and rejoined with a hinge marked with angles. Adjustment to the marked angle will produce any desired angle up to a maximum of 180°.

See also

• Polydrafter
• Special right triangles