theremin, electronic musical instrument invented in 1920 in the Soviet Union by Leon Theremin (also called Lev Termen). It consists of a box with radio tubes producing oscillations at two sound-wave frequencies above the range of hearing; together, they produce a lower audible frequency equal to the difference in their rates of vibration. Pitch is controlled by moving the hand or a baton toward or away from an antenna at the right rear of the box. This movement alters one of the inaudible frequencies. Harmonics, or component tones, of the sound can be filtered out, allowing production of several tone colours over a range of six octaves. The American composer Henry Cowell and the French-American composer Edgard Varèse have written for the theremin. The instrument was used in recordings by the American rock group the Beach Boys and in the soundtracks of several science fiction films.