A triangle is a musical instrument made of chromium-plated steel. It is a rod bent into a triangular shape with equal sides.

It is held by a loop attached to the top angle of the triangle. One end of the triangle is left open with the ends nearly touching. It is struck with a metal rod called a beater.

When it is struck it gives a high-pitched, ringing tone.

Although it seems a very simple instrument, it can be used to create a variety of sounds, as the video shows. When you open and close your hand on it, it tends to produce a more muted sound. This can be used to make a beat. If a very quiet sound is needed, then a much lighter beater (thinner rod) is used, sometimes something as thin as a knitting needle.