Which is the rarest instrument?

The theremin is widely considered to be the rarest musical instrument in the world. It is an electronic instrument that is played by moving your hands in the proximity of two antennas. The theremin was invented in 1919 by Leon Theremin and was popularized by Clara Rockmore in the 1930s. It is a notoriously difficult instrument to master, and there are only a handful of professional theremin players in the world.