Tuning Fork, a small U-shaped piece of steel that, when struck, produces a clear tone of unvarying pitch. There is a handle on the curved part. The prongs of the fork vibrate alternately toward and away from each other. The pitch depends on the rate of vibration, which depends on the length and thickness of the prongs. A tuning fork is used for adjusting the tone of a musical instrument and finding a standard pitch, usually A above middle C. It is also used for studying sound in the physics laboratory.