A tuning fork is an in the form of a two-pronged with the formed from a U-shaped bar of metal (usually ). It at a specific constant when set vibrating by striking it against a surface or with an object, and emits a pure musical tone after waiting a moment to allow some high to die out. The pitch that a particular tuning fork generates depends on the length of the two prongs. Its main use is as a standard of pitch to tune other musical instruments.

The main reason for using the fork shape is that it produces a very , with most of the vibrational energy at the , and little at the (), as is the case with other resonators. The reason for this is that the frequency of the first overtone is about 5²/2² = 25/4 = 6 1/4 times the fundamental (about 2 1/2 octaves above it). By comparison, the first overtone of a vibrating string is only one octave above the fundamental. So when the fork is struck, little of the energy goes into the overtone modes; they also die out correspondingly faster, leaving the fundamental. It is easier to tune other instruments with this pure tone.

Another reason for using the fork shape is that, when it vibrates in its principal mode, the handle vibrates up and down as the prongs move apart and together. There is a (point of no vibration) at the base of each prong. The handle motion is small, allowing the fork to be held by the handle without damping the vibration, but it allows the handle to transmit the vibration to a (like the hollow rectangular box often used), which amplifies the sound of the fork. Without the resonator (which may be as simple as a table top to which the handle is pressed), the sound is very faint. The reason for this is that the sound waves produced by each fork prong are 180° out of with the other, so at a distance from the fork they and largely cancel each other out. If a sound absorbing sheet is slid in between the prongs of a vibrating fork, reducing the waves reaching the ear from one prong, the volume heard will increase.

Currently, the most common tuning fork used by musicians sounds the note of , which has long been used as a standard tuning note by orchestras, it being the pitch of the violin's second string played open, the first string of the viola played open, and an octave above the first string of the cello, again played open. However, they are also commercially made to vibrate at frequencies corresponding to all musical pitches within the central octave of the piano, and other pitches.