What is the name of the small metal bar instrument used to tune a piano?

A tuning fork is the small metal bar instrument used to provide a reference pitch when tuning a piano.

A standard musical tuning fork is commonly made to vibrate at 440 hertz, producing the note A4. Piano technicians can use that reference to establish the temperament and tune the remaining strings through carefully measured intervals.

The fork’s two-pronged shape produces a stable pitch because most of its vibration is concentrated in the prongs rather than in a flexible handle. Striking it against a soft surface and placing the handle on a resonant object makes the sound easier to hear.

Modern tuners often use electronic devices instead, but the tuning fork remains an important symbol of pitch accuracy. A pitch pipe differs because it contains a set of tuned air reeds rather than a vibrating metal fork.