# triangle

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triangle
/truy"ang'geuhl/, n.
1. a closed plane figure having three sides and three angles.
2. a flat triangular piece, usually of plastic, with straight edges, used in connection with a T square for drawing perpendicular lines, geometric figures, etc.
3. any three-cornered or three-sided figure, object, or piece: a triangle of land.
4. a musical percussion instrument that consists of a steel triangle, open at one corner, that is struck with a steel rod.
5. a group of three; triad.
6. a situation involving three persons, esp. one in which two of them are in love with the third.
7. (cap.) Astron. the constellation Triangulum.
[1350-1400; ME < L triangulum, n. use of neut. of triangulus three-cornered. See TRI-, ANGLE1]

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Geometric figure with three sides and three angles.

Each two sides meet at a point called a vertex, and the three angles sum to 180°. A triangle with one 90° (right) angle is a right triangle. A triangle with all sides (and thus all angles) equal is equilateral, one with two sides equal is isosceles, and one with no two sides equal is scalene. Triangles are particularly useful in surveying, astronomy, and navigation. Two observation points (sight lines) form a triangle with a reference object serving as one vertex and the observation points as the other two. Knowing the angles of the sight lines and the distance between the observation points allows the calculation of the lengths of the other sides using the methods of trigonometry.

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percussion instrument consisting of a steel rod bent into a triangle with one corner left open. It is suspended by a gut or nylon loop and struck with a steel rod. It is theoretically an instrument of indefinite pitch, for its fundamental pitch is obscured by its inharmonic partials (component tones). Some players, however, perceive a suggestion of pitch and often possess more than one instrument. A single stroke on the triangle clearly penetrates the full force of an orchestra, and it is perhaps most effective when used sparingly.

The triangle was known by the 14th century and was sometimes trapezoidal in form; until about 1800 it often had jingling rings. With cymbals (cymbal) and bass drums (bass drum), triangles were basic to the Turkish Janissary music (Janissary music) in vogue in 18th-century Europe, entering the orchestra at that time as a device for local colour. In the 19th century it began to be used purely for its sound, as in Franz Liszt's (Liszt, Franz) Piano Concerto No. 1 in E-flat Major (Triangle Concerto).

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Universalium. 2010.

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