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<- 20afdo 21afdo 22afdo ->
Prime factorization 3 × 7
Fifth 32/21 (729.219c)
Dual sharp fifth 32/21 (729.219c)
Dual flat fifth 31/21 (674.254c)

21afdo (arithmetic frequency division of the octave), or 21odo (otonal division of the octave), divides the octave into 21 parts of 1/21 each. As a scale it may be known as mode 21 of the harmonic series or the Over-21 scale.