Dominant functioning as a Subdominant!

Question

The D9 chord voices appropriate seems to be able to function as a subdominant chord. This is very intriguing and I've always thought and read that dominant chords function as dominant chords(except in non-functional harmonies).

The notes(l2h) F# D A C E is a D9 chord. Playing it on guitar then going to a D7 then to G sound's functionally identical to C D7 G or Am D7 G. Yet it is really a D9 D7 G.

One can see this chord as a first inversion Am/D polytonal chord which could explain why it sounds like it is functioning as a subdominant but it seems like a stretch.

Anyone have any solid theoretical explanation for this chameleon chord and possible other examples?

Answer 1

The function of a chord largely depends on the context in which it is presented. I am going to assume that your comments are with respect to the key of G as it was the example that you used.

You are correct, in root position the chord would be: D F# A C E. Going to a "D7" on the guitar would be removing the 9th from the chord, thus you would have: D F# A C. If you were to go to a piano and play these chords, you would find them nearly identical - and you should, since it's literally almost exactly the same chord. Proceeding to a G major chord yields an authentic cadence.

From this point onward, I disagree with your statements. A progression of D7(9), D7, G does not sound functionally identical to C, D7, G or Am, D7, G progression.

From a roman numeral standpoint, the first progression would be: IV-V7-I

The second progression would be: ii-V7-I.

Let's compare the chords side-by-side:

• D F# A C E
• C E G
• A C E

As you can see, the C major triad shares two notes with D7(9), and the A minor triad shares three notes.

So why the confusion?

The answer I believe is two-fold:

• It is implied harmony.

Because the C major shares two notes, it can be used as a pivot chord from the D7(9). This would be useful for creating a plagal cadence in G major (IV-I). In which case, the chord could be re-categorized as a Cadd9(#11,13) chord instead, but this is very fussy. Speaking in a strictly chordal sense, implied harmony is the sense of harmonic motion (either progression or regression) through the use of either upper-tertian harmony or substituted chords. It is most often used in the harmonization of melodies; where the construction of a melody may imply certain harmonic realizations and cadences. This is of course in part contributed to by the expectations of listeners.

• It is the way you are voicing the chords.

If you voiced the chord such that the "E" was the top note (like you did in your question) and then proceeded to a D7 chord (assuming you are using the standard 1st position guitar voicing - putting the "F#" on the highest string - then you would have the sound of the leading tone. The reason why the "F#" from the voicing of the D7(9) doesn't suggest a cadence so strongly is for a couple of reasons: 1.) the harmony is more rich, and 2.) it is displaced by a couple octaves relative to the top string. Notes that are farther apart sound less dissonant.

In summation: I think you are being tricked by the extended harmony and your own voicings of the chords.

Addendum: In order for the original chord to serve as a sub-dominant, it would need to be followed by an E7 chord (E, G#, B, D) which would then of course resolve to A major.

Also, I strongly disagree that the D7(9) would bee seen as a polytonal chord. That term is used for harmonies that are typically non-diatonic. For example, if you played "F#" | "E" major.

Hope this helps.

{Edit} - It should also be noted that the crux of this answer is to address the aural confusion related to hearing a V7 as a IV chord in a simple functional harmonic progression.

Answer 2

D9 is an extended tertian chord. Extended chords have only become quite popular in the last hundred years or so because they're somewhat vague functionally, as you've pointed out. Often, extended chords can be thought of as polychords in that different parts of the chord can function differently. For example, in the key of G, your D9 can come nicely come after Amin and precede Dmaj because the F#-D-A part of D9 follows Amin like a normal Dmaj chord would and the C-E or A-C-E precedes Dmaj like a normal C-E or A-C-E would. But these connotations do depend on voice-leading.

In fact, D9 would work even better in this poly-functioning way if one part landed on the Dmaj part of the D9 in a contrapuntally independent way from a second part that lands on the Amin or C-E part.

Try this example on the keyboard:

This exercise emphasizes the phenomena I heard on your guitar example. You can probably think of numerous other examples like this where a chord based on the dominant (or whatever) can have other notes attached that function like the subdominant (or whatever) in a coherent way. Of course, there are other ways to use and hear D9 and the other extended chords.

Answer 3

As already said, "A progression of D7(9), D7, G does not sound functionally identical to C, D7, G or Am, D7, G progression". Probably, they sound you identically because of a lack exercise. Pay attention to them, put them in a context, and you'll find them tremendously different.

I think that there are no correct ways to explain why two different chords sound equal. For this same reason, it is a bigger nonsense ask why a progression sounds like TWO other progressions: do you think that Am and C sounds identical?? Please, listen to them good :)