The theremin is a completely electronic musical instrument invented in 1919 by physicist Lev Termen (whose name was later changed to Leon Theremin). It is often described as "the only musical instrument that is played without being touched." About 500 of the original models were manufactured in the 1920s.
A theremin has a vertical antenna which controls pitch and a horizontal antenna which controls volume. As the hand moves toward or away from the pitch antenna, the pitch ascends and descends smoothly and continuously, like a slide whistle or a musical saw. The theremin's sound is a pure sine wave.
Although the theremin has its devoted fans, most listeners find its sound eerie and not particularly musical. It has been treated mostly as a novelty or a sound effect. Many science fiction movies have used theremins to evoke a futuristic or outer-space quality. The theremin is also occasionally used in popular music, perhaps most notably in the Beach Boys' "Good Vibrations".