Choosing a diminished 7th over a dominant

Over time, my taste changes somewhat and I find myself gravitating toward new ideas. I can point out a few in the recent arrangement of “Honey in the Rock,” but I want to talk about one in particular.

Check out these examples and then we will talk about them.

If I asked you to name these chords, I would bet that most of you would call them some version of a diminished chord, maybe a viidim. The top one could be called a lot of things such as C#dim or C#dim/F# or even F#dim7 (the missing third does not really change anything). The bottom example is essentially the same chord but the B natural is a common color note in that diminished chord (really any of the notes a half step under the primary notes of a diminished 7th are color note candidates).

Really though, I would not call these chords diminished at all. They are really just V7s without the root. Technically, I would look at the first as V7(b9) and the second as V7(b9,13).

You can test my assertion by going to the piano and playing through these bars changing the F# in the bass to a D. What you will hear is that while the sound is different, the chord quality is exactly the same. Try it if you don’t believe me.

This is why the viidim chord and the V chord are considered functionally equivalent. For all intents and purposes, they are just the same chord with a different color.

So why choose one over the other? It is simply a stylistic choice, but lately I have been leaning toward using rootless dominants. They sound lighter and more subtle. By the way, I am not talking just about V7 chords but all secondary dominants.

Sometimes there are even more compelling reasons to do this substitution. For example, let’s talk about smoothing out the baselines. Here is a very obvious application of that using a simple I – ii chord progression. The first thing we are going to do is insert a V/ii in front of the ii. Of course, you can do this a lot and I have already talked about that here many times.

Original: I – ii
Step 1: I – V/ii – ii

Now here is the cool thing. A V/ii is a VI7 chord and if you remove the root from that chord, you get a diminished chord built on a #I. If you then sub #Idim for V/ii, you get this:

Original: I – ii
Step 1: I – V/ii – ii
Step 2: I – #Idim – ii

Note the smooth bass movement created by that. In fact, you can take it further with a common longer progression like this:

Original: I – ii – iii – IV – V
Step 1: I – V/ii – ii – V/iii – iii – IV – V/V – V
Step 2: I – #Idim – ii – #IIdim – iii – IV – #IVdim – V

After step 2, you have a long beautiful baseline moving in half steps.

Got it? Smoothing out bass lines is a great application of this idea but sometimes, you will just like the sound better. Either reason is just fine.