Weekly Music Theory Challenge 8/9/21

Show off your theory chops with my weekly challenge! You’ll find a new question here every Monday. Please comment to post your reply.

This week’s challenge:

What kind of scale produces both an A-flat dominant seventh chord and a B half-diminished chord (B minor seventh-flat fifth). Hint: add all the notes together!

Reply to post your answer, and check back on Friday, August 13th to see if you’re right!

ANSWER for 8/9/21

This is the OCTATONIC scale, also called the half step/whole step diminished scale. It’s built from alternating half steps and whole steps, for a total of eight different pitches within the octave:

This scale opens up harmonic avenues that are not possible with diatonic major or minor scales. Here are the chords from this week’s theory challenge:

The 20th Century French composer Olivier Messiaen called this scale a mode of limited transposition. Unlike major or minor scales, it can only be transposed a limited number of times before you end up with the exact same pitches. Major and minor scales can be transposed to every one of the twelve chromatic pitches within the octave, each giving a different set of notes. But the half-whole-half octatonic can only be transposed three times before all the pitches repeat. So the octatonic scale above, starting on B, has the same pitches as the ones starting on D, F, and A-flat. Notice those four starting notes–B, D, F, A-flat–spell out a diminished seventh chord, a harmony readily available in the octatonic scale.

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