The Dominant Seventh Chord
Creating the Dominant Seventh and Secondary Dominant Seventh Chords
When chords are identified as "Dominant" chords, that is to say, chords that appear on the tone that is a fifth higher than the tonic, it is commonplace to add an additional note by stacking a minor third above the triad. This turns the dominant triad into a Dominant Seventh Chord.
We will explore this in a more detail when we get to part-writing, but at the moment, simply observe that adding an additional tone to the dominant triad creates a dominant seventh chord. The Dominant Seventh Chord contains a minor seventh above the root (G to F), which is also a minor third above the top note in the triad (D to F):
What is important about the dominant seventh chord is to note the existence of a tritone between the third and seventh degrees of the chord (the now-familiary tritone between B and F). It is the grafting of the tritone onto what was a simple major triad that gives the dominant seventh chord its unique sound. In common practice classical music, this "unstable" element, the tritone, required resolution. This will be explored in more detail on a subsequent page. For now, simply observe the structure and know that the dominant seventh sound further propels the V chord towards its resolution on the I triad.
The Dominant of the Dominant (V7/V)
Similarly, we can use the secondary dominants listed on the previous page and create dominant sevenths out of them, all within the key of C. Notice that when we do that, we must in each instance create a minor seventh above the root, or a minor third above the top note of the triad. Here is the V/V (the "Five of Five" or the "Dominant of the Dominant") that is turned into a V7/V ("Dominant Seventh of the Dominant") by adding a C above the D major triad.
Here, with the G chord as the dominant in the key of C, the D major triad becomes the V of V or the Dominant of the Dominant. Adding the seventh above, the high C, which is a minor seventh above the D root, creates a "V7 of V" or "the Dominant Seventh of the Dominant." Note that, if you think of the cycle of fifths, it takes on the F# from the key of G.
The Dominant of the Subdominant (V7/IV)
We can use this same process with the dominant of the subdominant, we can take the tonic triad, add a minor seventh above the root (you can also say—a minor third above the fifth of the triad) and create a dominant seventh chord that points strongly down to the IV chord. Clearly, this is a "V7 or IV" or a dominant seventh of the subdominant.
Notice here that the tonic triad is already a fifth above the subdominant, and thus by its very nature it is the dominant of the subdominant—tonic always has a tendency to move towards subdominant. To create the dominant seventh of the subdominant, the C triad is given a lowered seventh, a minor seventh above the root. Note here too that, if you think of the cycle of fifths, it takes on the Bb from the key of F. Thus the C chord becomes the Dominant seventh of the F chord, or the Dominant seventh of the Subdominant (the V7 of IV).
The final fundamental chord: The relative Tonic (vi)
There remains one final chord to discuss, at which point you will havea great deal of the vocabulary of both popular and classical harmony! This is the vi chord (the "six chord"), which can also be thought of as the relative tonic.
The term "relative tonic" is used because the six chord often substitutes for tonic—it serves as a stand-in for the tonic triad—due to its sharing of two common tones with the tonic. However, it is a significant modal and expressive shift because the vi chord is minor when the tonic triad is major.
So often in tonal music in a major key, the shift from a passage of music using I, IV and V into a passage that begins with a vi chord has a very poignant and expressive quality. Here this beginning of the Db nocturne of Chopin. Can you hear when the second section begins, how there is the sift to the relative tonic?
Summary
We now have the following chords at our disposal:
- The tonic triad (T or I)
- The dominant triad (D or V)
- The subdominant triad (S or IV)
- The dominant triad of the dominant (DD or V of V)
- The subdominant triad of the subdominant (SS or IV or IV)
- The dominant seventh chord (V7or D7)
- The dominant seventh chord of the dominant (V7/V or D7D)
- The dominant seventh chord of the subdominant (V7 of V or D7(S)
- The relative tonic (vi or T-R for "tonic-relative")
Here is how they look, stacked up in the key of C:
OK, this is a lot of seemingly abstract stuff. But with these harmonies in your mind, let's put them into your ear. You'll see that you've been hearing these harmonies all your life, you just never had the terms for them. The next page will present these harmonies through the vehicle of some basic popular tunes.
A last concept: modal shift of the tonic to minor
Before we move on, one other piece of the puzzle: often, for expressive purposes, the I chord, the tonic triad, can shift to the minor version by lowering the third. C major, C-E-G, becomes C minor (C-Eb-G). This is most wonderfully expressed in the opening to Richard Strauss' Also Sprach Zarathustra, his orchestral tone poem based on the writings of Friedrich Nietzsche. This passage was made world famous when it was used in the science fiction film 2001: A Space Oddysey:
This is a most effective description of a sunrise, made dramatic by a simple movement of root, fifth and octave, followed two modal shifts of the tonic triad: first from C major to C minor, then from C minor to C major.
On the next page, let's listen to some pieces from various genres that clearly demonstrate the basic movements of these harmonies.
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