A tuning fork is a sound resonator which is a two-pronged fork. The prongs, called tines, are made from a U-shaped bar of metal (usually steel). This bar of metal can move freely. It resonates at a specific constant pitch when set vibrating by striking it against an object. It sounds a pure musical tone after waiting a moment to allow some high overtone sounds to die out. The pitch depends on the length of the two prongs. Its main use is as a standard of pitch to tune other musical instruments, and in some tests of hearing.