A triangle is a metal bar percussion instrument of indefinite pitch made of steel that is bent into the shape of a triangle (hence the name), open at one corner, and produces a ringing sound when it is suspended on a string and struck with a metal or steel rod beater. There are many sizes and thicknesses of triangles
and the differences make sound pitch variances.
Triangles first became known in Europe during the 14th century and some were even trapezoidal in shape (and until about 1800 often had jingling rings strung on them).
During the 18th century, triangles were often used in Turkish Janissary (military) music and in orchestras. In the 19th century, triangles were utilized because of their unique sound.
The Hungarian composer Franz Liszt included a triangle solo
in his first piano concerto (1849), called the "Triangle" concerto.
Liszt Piano Concerto No.1:
Pianist: Martha Argerich
They are also standard rhythm instruments used in United States Acadian (Cajun) folk music.
La Danse de Mardi Gras
The Balfa Brothers
Traditional Cajun Music
Pieter Lastman: Orestes and Pylades Disputing at the Altar
(detail triangle player)
Vic Firth Percussion 101
How to Make Sounds With a Triangle
A Girl with a Marmoset in a Box, a Girl with a Triangle Sitting,
and a Boy with a Hurdy-Gurdy