The Monochord is strung on both sides. one side has 28 strings all tuned to the same note. This is the drone, the drone plays the harmonic overtones.
On the reverse side of the monochord are 13 strings which are tuned to a pentatonic scale in the same key as the drone.This is the Koto.
Both sides of the monochord can be palyed at the same time creating an orchestral effect of tune, drone and overtone.
Simply a wonderful way to meditate. Enjoy.