A tuning fork is a simple metal two-pronged fork with the tines formed from a U-shaped bar of elastic material (usually steel). When set vibrating with a blow from a mallet, and after waiting a moment to allow some high overtones to die out, a tuning fork resonates at a constant, specific pitch. The pitch that a particular tuning fork generates depends upon the length of the two prongs, with two nodes near the bend of the U.
When struck, it gives out a very faint note which is barely audible unless held right up to the ear. For this reason, it is generally struck and then pressed down on a solid surface such as a desk which acts as a soundboard and greatly amplifies the note.
Large forks are often made to be driven electrically, like an electric bell or buzzer, and can vibrate for an indefinite time. They are commonly used to tune musical instruments, although electronic tuners also exist, and, of course, some musicians have perfect pitch. Tuning forks can be tuned by grinding material off the tines (filing the ends of the tines to raise it or filing inside the base of the tines to lower it) or by sliding weights attached to the prongs. Once tuned, a tuning fork's frequency varies only with changes in the elastic modulus of the material; for precise work, a tuning fork should be kept in a thermostatically controlled enclosure.
A number of keyboard musical instruments have been made which use tuning forks as their sound source. None of them have ever been popular, although the Rhodes piano, which has hammers hitting constructions working on the same principle as tuning forks, is widely used.
A tiny quartz tuning fork is used in quartz digital watches. The piezoelectric properties of quartz crystals cause a quartz tuning fork to generate a pulsed electrical current as it resonates, which is used by the computer chip in the watch to keep track of the passage of time. In today's watches, they generally resonate at 32,768Hz. (See quartz clock.)
Tuning forks are also used therapeutically in sonopuncture. John Beaulieu, a researcher on the therapeutic benefits of tuning forks, has recorded an album of music made entirely with tuning forks called "Calendula."