We already know that the diminished chord, when located a semitone above the V7 chord has the same tritone. See this example, which is in the key of C major (G is the fifth degree):
- G#° chord notes: G#, B, D, F
- G7 chord notes: G, B, D, F
The tritone present in both is between the B and F notes (3 tones apart).
The diminished chord (G#°) has yet another tritone, between the G# and D notes. We could then think about changing the G7 chord to make it even more like the diminished chord. The only note that would be missing from the G7 chord to form this tritone (G# / D) would be the G# note (which is the ninth flat of G), so we can put a ninth flat on the V7 chord, forming G7(b9).
We now have two equivalent chords:
- G#° chord notes: G#, B, D, F
- G7(b9) chord notes: G, G#, B, D, F
Since we also know that G#° = B°, and B is the seventh degree of our tonality, we can generalize by stating that the VII° (diminished seventh degree) and V7(b9) chords are equivalent.
Note: Thinking about the seventh degree chord VII° instead of V#° is the most common.
Now notice the following:
- C is the chord of resolution of G (V7 – I).
- The flat ninth of G corresponds to the flat sixth of C (G#).
- This sixth flat of C is present in the C minor scale (the C major scale has a major sixth, not a minor sixth).
Conclusion: The G7(b9) chord is indicated as dominant to resolve in the C minor chord!
Well, then you already know that, if the idea is to place a V7 chord to resolve in the minor first degree, we can add an extension note (b9) to this dominant forming a V7(b9), as this will strengthen this cadence!
In this topic, we discovered one more reason that makes the ascending diminished chord resolve well in minor chords, as it is equivalent to a V7(b9).
To conclude this subject, try to practice these equivalent chords VII° = V7(b9) in the other tonalities, incorporating this concept into songs you already know. This will enrich your view of substitutions.
Go to: SubV7 Chord
Back to: Module 10