Dominant Seventh Chords and Secondary Dominants
The V7 Chord
We saw in the page on Polar Harmonies that it is a commonplace practice in all tonal music, whether Classical, Jazz, Gospel, Rock or any combination of the above, to add an additional tone to the triad on the fifth degree of the scale in order to create what we generically refer to as a dominant seventh chord. This is a chord with a major triad as its basis, with an added minor third above the triad (which is also a minor seventh above the root). This is what distinguishes the dominant seventh chord from all other seventh chord types: it is a major triad with an added minor seventh (a minor third above the triad):
This is the conventional view of the dominant seventh: a tonic triad with a minor seventh stacked on top.
The deeper perception, based on Polarity Theory, is that the dominant seventh chord is a combination of two poles: the dominant triad brought into relationship with the subdominant of the reciprocal fifth, with both poles pointing towards a resolution to the tonic triad:
The usefulness of this additional note— a practice which goes back to the 17th Century— is the tension that is created within the context of triadic music. By adding the minor seventh to the triad, we introduce a tritone between the third and seventh degrees of the chord:
It is common for this interval, this tritone, to create a sense of tension in need of resolution. That resolution is always in the direction of the scale tones. In other words, the seventh leads upwards to the tonic (as its name "leading tone" implies) and the the fourth falls downward to the third (recall that it was this tendency for the perfect fourth to fall into a third that made the perfect fourth a dissonance two-part modal counterpoint). Notice that these correspond to the places within the major scale where the half steps occur
If the tritone is inverted, the same resolution occurs, only now the resolution is inward instead of outward, as in shown in the second measure.
A great example to emphasize this point is to create a pattern or resolving dominant seventh chords around the cycle of fifths. If you have some keyboard skills, you should play this at the piano. If you are a music major or are serious about your growth as a musician, you should memorize this pattern and be able to play it on either the piano or the guitar (or both!).
The first example starts with the Tritone of the C7 chord voiced with the third (the E) on top and the 7th (the Bb) below. It resolves outward to the F triad (with the root on top and the third below). The root then drops down to become the 7th of the F7 chord, which is now going to become the Dominant Seventh chord in the key of B. This pattern continues all the way around the cycle. Note that at the Key of Db, we shift over and the Db triad becomes the C#7 triad in the key of F# major.
The second example reverses the pattern, with the tritone of the C7 chord voiced with the seventh on top.
Listen to the playback while studying these examples. You will be asked to write down this pattern, so it's important to have this in your ears.
Listen to this pattern so that you can clearly hear the resolution of V7 to I harmonies. Notice that here, the fifth of the chord is not played—often the fifth can be eliminated when resolving a V7 chord to the tonic chord—the most important notes are the roots moving downward by fifths/upward by fourths and the stepwise resolution of the tritone.
In Jazz harmony, the two notes of the tritone are called the "guide tones" as they guide the hand or the ear to the resolution of the next harmony.
There are a million musical examples that demonstrate the resolution of the V7 chord to the I chord. Here are a few simple ones within a variety of styles:
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Secondary Dominants
Secondary Dominants make use of the pattern around the cycle of fifths as demonstrated above. While remaining within one key, for example the key of C major, dominant seventh chords can temporarily be created to take advantage of the strong forward motion created by the movement of the dominant seventh chord to the triad a fifth below, with the proper resolution of the tritone. We have seen two examples of the secondary dominant chord already, on the page on polar harmonies. This was the V7 of IV and the V7 of V. Both of these chords move along the central spine and are a basic function of music, so basic that they can show up in a simple cowboy song like "Back in the Saddle Again."
Using V7/IV and V7/V (movement to major triads)
Here is the basic motion: the tonic chord takes on the lowered seventh from the subdominant key, becoming a V7 of IV. This resolves to IV. The upper voices of the IV chord (the A and the C) remain, while the rest of the chord changes to create a D7 chord, the V7 of V. This resolves to the V triad, which then picks up the seventh to resolve back to C. Here's the passage, with the Roman Numerals below and the Function Symbols in between the staves:
Using V7/vi and V7/ii (movement to minor triads)
It follows that secondary dominants can be created on other steps of the scale. These secondary dominants would resolve to minor triads, specifically the minor triads based on the the submediant (vi) and the supertonic (ii).
Clearly, these are called the V7 of vi and the V7 of ii. Each start with the triad that is a fifth above the destination chord (the vi and the ii) and turn these chords into dominant sevenths. You've heard this a million times in popular music and jazz. In the key of C they look like:
We can see the function symbols describe how the chords move. The V7/ii is the dominant of the relative tonic. The V7/ii is the dominant of the relative subdominant.
Summary
The dominant seventh chord occurs "naturally" — only using the notes of the major scale — on the fifth degree of the scale. In C, this forms the G7 chord, spelled G-B-D-F. This chord type is then employed on other steps of the scale to create secondary dominants.
The most basic secondary dominants are the ones that occur as V7s of the subdominant (V7 of IV) and as V7 of the dominant (V7 of V). Other secondary dominants can be formed above the vi and the ii chords, forming the V7 of vi and the V7 of ii.
These harmonies are employed liberally in a great variety of musics: in classical music certainly, all during the common practice period, but just as much in jazz and rock and popular (commercial) musics. See the page of musical examples to hear music that makes use of these harmonies.
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