60. Secondary Dominants (3)

Last time, we looked at secondary dominants by attaching the "V→I" resolving motion to the other major diatonic chords as well.
Along the way, the 5th and 7th chords sounded off in a way the others didn't, and this time we'll explain why.

To review last time: "The motion of the 7th chord is occasionally explained as a secondary dominant, but m7♭5 and dim chords are too unstable to serve as the I being resolved to, so it's common to consider that VIIdim and VIIm7♭5 basically have no secondary dominant."

We'll work in Key = C major.
Let's listen to it.

F#7→Bm7♭5

F#→Bdim

F#7→Bdim

V7/V

Next, let's check the 5th chord as well.

D7→G7

D→G

D7→G

Moving to G gave a sense of resolution, but if you're not familiar with blues and the like, moving to G7 may sound a little off.

Let's listen once more.

D7→G7

This chord makes a strong progression from II7 to V7, the dominant chord of the tonic's diatonic chords.
In other words, it's a secondary dominant chord that moves to the key's own dominant chord.
Because of this, its function and role differ slightly.
This time we'll dig into this 5th position.

About the double dominant (doppeldominant)

The 5th position has a special name.

Common names for it include double dominant and doppeldominant (Doppeldominante).
"Double" is English and "doppel" is German, and both mean something like "twofold."

Secondary dominants played the role of partially modulating and resolving to the next chord.

One example is a partial modulation that briefly goes to Key = F major, for instance.

V7/IV

C7→Fmaj7

C→F

C7→F

Let's tidy up the voicing of the V7/V from earlier and check it.

This II7 (V7/V) → V7 progression has the characteristic that the tritone moves down a half step in parallel and connects to the tritone of V7.
With four-note chords, both are dominant seventh chords, so tritones appear one after another.

The above is just the double dominant part in Key = C, but let's make the root move in strong progressions and connect dominant seventh chords smoothly.

Then the chords go D7→G7→C7→F7→B♭7→E♭7→A♭7→C#7 (D♭7)→F#7 (G♭7)→B7→E7→A7→and back to D7→G7→...

Does this order remind you of something?

That's right, it's the order of the circle of fifths.
You can also think of it as the dominant chords of each key lined up.
Using this, it seems we can modulate to various keys and finally settle down.
Let's actually try it.

The original key is C, but...

Chords: C→Am→Dm7→G7→C7→F7→B♭7→E♭

It settled in E♭ major.
Of course, with the knowledge we've covered so far, including harmonic and melodic minor,

Chords: C→Am→Dm7→G7→C7→F7→B♭7→E♭m

you can also settle in E♭ minor.

Next time, let's combine what we've covered so far with this lesson, put it to actual use and bring it into standard progressions.