Add flavor to your progressions with secondary dominants
It probably happened to you sometime that you were writing a chord progression, you played it and felt that it sounded like 90% of popular music, or maybe you felt that it lacked flavor, something that gave it that extra.
Here's a tool that can help you achieve that goal: secondary dominants.
Let's remember that there are two very important chords in a tonality: the tonic and the dominant. These correspond to the I and V chords in a tonality.
The tonic gives us a sense of rest; it is where we feel that the sound rests. The dominant chord is the one with the most tension and seeks to resolve the tonic; it creates instability.
In this way, in the major scale of C, for example, we would find that the tonic, subdominant, and dominant functions would be respectively:
What are the secondary dominant chords?
A secondary dominant is the dominant chord of the chord we want to reach. We call it secondary because they do not belong to the key we are playing, but they create tension for a moment to resolve one of the chords that does belong to the key. These are always major chords with a minor seventh, regardless of whether they resolve to a major or minor chord.
Secondary dominants are unstable chords and that is why they are usually placed in the weakest part of the bar.
If we harmonize the major scale, we see that the dominant chord is formed at the degree V (fifth interval right from the tonic).
The formula for this type of chord is: (1, 3, 5, 7b). As we see, there is an interval of tritone between the 3rd and 7thb; this makes this chord have a strong tendency to resolve over the tonic (this type of resolution is called authentic cadence).
The high tendency to resolve over the dominant chords makes any diatonic chord susceptible to having a dominant resolving over it. This type of resolution of dominant over a diatonic chord (except the 5th degree) is not considered a cadence as such; they are only resolutions of dominant. These types of dominants are called secondary dominants.
Let's analyze this chord progression: C - A7 - Dm7 - G7 - C
There seem to be two tonalities because there are two different dominant 7th chords; in diatonic harmony, the dominant 7th is the most conclusive clue to the tonal center, because it is always a V chord in major tonalities, and almost always in minor ones. According to this analysis, the C major tonality is indicated by the G7 chord, and the D minor tonality is indicated by A7. However, the Dm7 chord is also the IIm7 chord of C. Does A7 indicate a change in key, or is there another explanation?
To the ear, it will seem that the tonal center of the entire progression is C. The change in the quality of the A chord increases the sense of anticipation that Dm7 is coming, but does not indicate the presence of a new tonality. A7 is an example of Secondary dominance.
Although the Secondary Dominant seems to break the rule that the 7th dominant functions as a V, the rule still applies. The Secondary Dominant chord still functions as a V, but it is the V chord of another chord other than the I. In the example, A7 is the V7 of IIm7. This analysis shows that C is still considered the tonic, G7 is the primary dominant in the key of C, and the function of A7 is to present Dm7 in a more dramatic way than the diatonic VIm7, Am7 would. The function of A7 is written as "V7/II", clarifying the function within the progression. Therefore, the complete analysis of the progression remains as follows:
C = I
A7 = V7/II
Dm7 = IIm7
G7 = V7
C = I
Secondary dominants occur in major and minor shades. The general rule is this:
Any diatonic chord can be preceded by its secondary dominant except the VIIth chord in major and the IInd chord in minor.
The exclusion of the VIIº and IIº chords is because these chords are based on a diminished triad, which is considered too dissonant to function even temporarily as a resolution point.
As the role of the secondary dominant is to increase the sense of anticipation of the next chord, the resolving chord must be able to stand on its own, or the fluidity of the progression will be broken.
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