Hand percussion, idiophone
Single note, open and closedhigh
The triangle is an idiophone type of musical instrument in the percussion family. It is a bar of metal, usually steel but sometimes other metals such as beryllium copper, bent into a triangle shape. The instrument is usually held by a loop of some form of thread or wire at the top curve. It was first made around the 16th century in England.
On a triangle instrument, one of the angles is left open, with the ends of the bar not quite touching. This causes the instrument to be of indeterminate or unsettled pitch. It is either suspended from one of the other corners by a piece of, most commonly, fishing line, leaving it free to vibrate, or hooked over the hand. It is usually struck with a metal beater, giving a high-pitched, ringing tone.
Although today the shape is generally in the form of an equilateral triangle, early instruments were often formed as non-equilateral isosceles triangles. In the early days the triangles did not have an opening and had jingling rings along the lower side.
The triangle is often the subject of jokes and one liners as an archetypal instrument that seemingly has no musical function and requires no skill to play. (The Martin Short character Ed Grimley is an example.) However, triangle parts in classical music can be very demanding, and James Blades in the Grove Dictionary of Music and Musicians writes that "the triangle is by no means a simple instrument to play". In the hands of an expert it can be a subtle and expressive instrument that makes a beautiful tone/pitch with the vibration.
A triangle roll, similar to a snare roll, is notated with three lines through the stem of the note. It requires the player to quickly move the wand back and forth in the upper corner, bouncing or "rolling" the wand between the two sides.
In European classical music, the triangle has been used in the western classical orchestra since around the middle of the 18th century. Wolfgang Amadeus Mozart, Joseph Haydn and Ludwig van Beethoven all used it, though sparingly, usually in imitation of Janissary bands. The first piece to use the triangle prominently was Franz Liszt's Piano Concerto No. 1, where it is used as a solo instrument in the third movement, giving this concerto the nickname of "triangle concerto". In Romantic era music, the triangle was used in some music by Richard Wagner, such as the "Bridal chorus" from Lohengrin. Johannes Brahms uses the triangle to particular effect in the third movement of his Fourth Symphony. The triangle is used extensively in Hans Rott's Symphony in E major, particularly in the BIS recording; in later recordings, the conductor has reduced its role. 
Most difficulties in playing the triangle come from the complex rhythms which are sometimes written for it, although it can also be quite difficult to control the level of volume. Very quiet notes can be obtained by using a much lighter beater: knitting needles are sometimes used for the quietest notes. Composers sometimes call for a wooden beater to be used instead of a metal one, which gives a rather "duller" and quieter tone. When the instrument is played with one beater, the hand that holds the triangle can also be used to damp or slightly modify the tone. For complex rapid rhythms, the instrument may be suspended from a stand and played with two beaters, although this makes it more difficult to control.
A triangle played latin style, opening and closing the hand for rhythmic effect.
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In folk music, forró, cajun music and rock music a triangle is often hooked over the hand so that one side can be damped by the fingers to vary the tone. The pitch can also be modulated slightly by varying the area struck and by more subtle damping.
In the Brazilian music style Forró it used together with the zabumba (a larger drum) and an accordion. It forms together with the zabumba the rhythmic section. It provides usually an ongoing pulse, damping the tone on the first second and fourth while opening the hand on the third beat to let most frequencies sound. It can be used though extensively for breaks and to improvise to vary the rhythm. A notable instrumentalist in this style has been Mestre Zinho.