A triangle is a percussion instrument that consists of a steel rod that is usually bent into an equilateral triangle shape, except that one corner is left slightly open to give it an indefinite pitch. It is suspended from a hook by a thread or wire and struck with a steel rod. The triangle is usually used sparingly only as an accent, as a single stroke can be heard clearly above the full force of an orchestra.
In spite of its great simplicity, the triangle still has some versatility. For example, striking the outer sides produces a louder volume, while striking the lower end of the closed side produces a softer volume. The size of the striking rod can also affect the sound, and a tremolo effect can be produced by positioning the rod near a closed corner and hitting both sides rapidly.
Similar instruments may have been in use since ancient times, although the shape was not necessarily triangular, and an early version was mentioned in a tenth century European manuscript. The triangle came into use by Western classical orchestras from around the middle of the 18th century and was employed in some leading works at that time, including Mozart's operas and Beethoven's Ninth Symphony.