The document discusses the theremin, an electronic musical instrument played without physical contact. It was introduced in the 1920s and gained popularity in the US. The theremin is tuned over three and a half octaves and its operation is based on beat frequencies. Leon Theremin, its inventor, stayed in the US where he met Clara Rockmore, the first virtuoso player. Theremin later developed other electronic instruments. In the 1950s, Robert Moog started producing the theremin commercially and in kit form, increasing its popularity. The theremin was featured in several movies and used by bands like The Beach Boys and Led Zeppelin.