Why can a diminished chord replace a dominant 7th (V7) chord?
Asked by Tom ·
In Example 4 of the Rhythm Changes Soloing Concepts lesson, the G7 is replaced by a Bdim. What's the idea behind that substitution? Isn't the tritone substitution for G7 an Ab7?
Emil Ernebro answers The B diminished is actually a very common substitution for the G7. We are going from Bbmaj7 → G7 → Cm7 → F7, and the Bdim creates a chromatic bass line from Bb → Bdim → Cm7.
The Bdim is actually very similar to the G7, but we get a bit stronger "pull" towards the Cm7. The Bdim (or rather Bdim7) is the same as playing a G7(b9). That b9 note (Ab) in the Bdim7 chord creates even more tension than a regular G7 chord would, so it sounds great to substitute G7 with the Bdim (or Bdim7).
In the next chords of the lesson, Eric uses the C#dim to get to the Dm7. This is the exact same idea. The Dm7 is a substitution for Bbmaj7, and we could think of the C#dim (which is a half step below the next chord, Dm7) as an A7(b9) chord, just like we think of the Bdim7 as a G7(b9) chord.
The general rule: whenever you have a V7 to I situation, in other words a dominant 7 chord that leads "home" (a fourth up, or a fifth down, which is the same thing), you can play a diminished chord from the 3rd of the dominant chord. For example, instead of G7 → Cm7, play Bdim7 → Cm7 (or Cmaj7). This creates more tension over the dominant chord, and it doesn't matter if the I chord is major or minor.
So this case is not a flat five substitution, but later in the example Eric does use the flat five substitution. The next part of the progression is Dm7 → Db7 → Cm7 → B7: the Db7 is a flat five substitution for G7, and the B7 is a flat five substitution for F7.
