Stagg 4 inch triangle with beater.

The triangle is an idiophone type of musical instrument in the percussion family. It is a bar of metal, usually steel in modern instruments, bent into a triangle shape. 

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 pitch. It is either suspended from one of the other corners by a piece of thin wire or gut, 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 the shape is nowadays generally in the form of an equilateral triangle, early instruments were often formed as isosceles triangles. Some triangles have jingling rings along the lower side. 

from www.wikipedia.org