Overtone flute refers to a type of a flute which lacks playing holes, and so can only produce notes from a natural scale. Lacking holes, the tube of such a flute has much higher resonance Q factor than a tube of standard flutes,Jean-Marc Bonard, The physicist's guide to the orchestra. European Journal of Physics, 22 (2001) 89–101. PII: S0143-0807(01)16272-0 and so can produce dozens of upper harmonics (rather than 2–3 for recorder, or five for kaval), making it possible to play simple melodies (see resonance for explanation of the effect).