At 6:38-ish in

The guy says he replaced the Gm with the leading tone diminished chord (of Bb major), but I don't get how he got that to be B-diminished?

Isn't the leading tone diminished of Bb major A-diminished? (Because A is the leading tone of Bb-major scale)

Who is wrong here? Him or me?

2 Answers 2


You're both correct, but the person in the video should have been more clear. He should have said that the B diminished seventh chord is the leading-tone chord of the ensuing C-minor chord.

Because it's viio7 of C minor, that Bo7 functions to briefly make C minor sound like tonic even though the piece as a whole is in B-flat.

  • Ah, I guess he is not using the natural minor then! I don't why, but I always neglect the harmonic and melodic minor. Jul 31, 2020 at 8:30
  • I hear it more as replacing a diatonic vi with a secondary dominant to the ii as opposed to a new tonic. Jul 31, 2020 at 20:16
  • 1
    @JohnBelzaguy I agree 100%, that's exactly what it's doing. But the logic of a tonicization is that it briefly points to a new tonic.
    – Richard
    Jul 31, 2020 at 20:19
  • True, the leading tone resolving makes a non tonic chord sound more important...or more something! Jul 31, 2020 at 20:29

You are right that the leading tone diminished chord of Bb is A diminished but in this case he is talking about using the leading tone diminished chord of the Cm chord which is B diminished.

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