The triangle is a musical instrument in the percussion family, classified as an idiophone in the Hornbostel-Sachs classification system. Triangles are made from a variety of metals including aluminum, beryllium copper, brass, bronze, iron, and steel. The metal is bent into a triangular shape with one open end. The instrument is usually held by a loop of some form of thread or wire at the top curve, to enable the triangle to vibrate and it is struck with a metal rod called a "beater". The triangle theoretically has indefinite pitch, and produces a plurality of overtones when struck with an appropriate beater.