Bisector (music)

In diatonic set theory, a bisector divides the octave approximately in half (the equal tempered tritone is exactly half the octave) and may be used in place of a generator to derive collections for which structure implies multiplicity is not true such as the ascending melodic minor, harmonic minor, and octatonic scales.

Well formed generated collections generators and bisectors coincide, such as the perfect fifth (circle of fifths) in the diatonic collection.

The term was introduced by Jay Rahn (1977), who considers any division between one and two thirds as approximately half (major third to minor sixth or 400 to 800 cents) and who applied the term only the equally spaced collections.

Clough and Johnson both adapt the term to apply to generic scale steps.

However, five steps in the octatonic scale alternates between 7 and 8 semitones, so it is a bisector and not a generator: This music theory article is a stub.