The theremin (named after its inventor, Léon Theremin) is played without physical contact; sound is produced by the movement of hands in the proximity of two antennae. Frequency (pitch) is manipulated by the distance of a hand from one antenna, while amplitude (volume) is controlled by the distance of the other hand to the second antenna.

Apparently, the theremin is simple to learn, but mastery is difficult. There is no feedback guidance; no valves, keys, frets, or fingerboard positions. Another challenge is minimizing portamento; the carriage, or slurring, when sliding from one frequency to another.

Have a listen to Lydia Kavina* playing Claire de Lune 

*Lydia Kavina is the grandniece of Léon Theremin.