Tuning forks are manufactured to vibrate at specific frequencies. According to The Physics Classroom, when a fork’s tines stretch after being struck, they compress air molecules into a small space and then release them as the tines return to their original positions. These areas of compression and refraction form a sound wave that transmits the pitch of the tuning fork through the air or through a solid, such as wood.
The pitches produced by tuning forks are used to tune musical instruments. Forks of different sizes produce different pitches. Tuning is complicated when working with instruments such as pianos, where many pitches must be checked. It is, perhaps a little ironically, much easier to tune an entire orchestra because each instrument is attempting to reach the same, single pitch.
To tune an orchestra, a concert master strikes an A440 tuning fork in the air so that musicians tune to its pitch by ear. Tuning a piano is a bit different. Because tuning forks transmit sound to the wooden cabinet of a piano, a tuner is able to place the fork’s base against the piano and listen to its pitch while adjusting one string until the tuning fork’s pitch and the string’s pitch match.