Kokū is considered one of the oldest compositions for shakuhachi, one of the three fundamental pieces together with Kyorei and Mukaiji.
Koku means “Empty Space,” or “Empty Sky,” (“Ko,” “empty”, and “Ku” “space”, or “sky”.)
There are many legends about this piece, and many different versions. Some versions are based on the story of the “Bell Ringing in an Empty Sky”. It refers to the death of the monk Fuke and his disappearance from his coffin. Only remains the sound of his bell coming from the empty sky. Other legends say that this piece has been composed in the 12th century by the monk Kyochiku, who heard this melody in a mystical dream. With the mist blocking the moon, it looked like the sound of the flute was coming from an empty sky.
Koku has a flavor of infinite mist and echoes. It is said that playing this piece helps one explore the boundaries of “mu” or nothingness, transcending reifications, the artificial cognitive boxes into which we place objects, situations and emotions.
Source: The International Shakuhachi Society