A secondary dominant is any chord that has the dominant function over another chord that is not the tonic of the song.
For example, in the key of C major, the dominant chord is G7. If, in that tonality, the A7 chord appeared, that chord would be a “secondary dominant”, since it is a dominant that resolves in D, not in C (our tonic in this case).
Note that secondary dominants are not part of the natural key. They are auxiliary chords, they only serve to “prepare” a progression for some other degree of the key.
A possible synonym for secondary dominant is “auxiliary dominant“, but the latter is most often used in the context of Borrowed Chords (a subject that we will see in other topics). This helps to make a distinction between the purpose of these dominants, and here in Simplifying Theory we will make this differentiation to facilitate understanding.
How to use Secondary Dominants
Secondary dominants are often used to anticipate the natural dominant of the song. For example, in the previous case, the natural dominant of the song was G7, so we could play another dominant before it to prepare going into G. Observe:
G’s dominant is D7. So, we would have the sequence | D7 | G7 | C |, where D7 is the secondary dominant. This dominant is also called “dominant of the dominant“, since it serves as dominant for another dominant.
In terms of nomenclature, it is customary to use the notation V7/V7 or V7/V to highlight that it is a secondary dominant for another dominant (of the fifth degree). If you were, for example, a secondary dominant preparing for the fourth degree, we would write V7/ IV.
Application
Very well, the concept of a secondary dominant is already clear. Now we are going to show the implications that this concept can have. As the dominant V7 is always a fifth above the chord it is going to resolve, we can “play” with successive circles of fifths. In the previous case, we played D7 before G7, but we could also play A7 before D7 and E7 before A7, forming the following sequence:
| E7 | A7 | D7 | G7 | C |
This sequence is one preparation after another, which was resolved only at the end in C. First, E7 prepared for A, but A was in the seventh, preparing for D, and so on until ending in C. This type of progression is widely used in jazz.
As we have already seen, these are “extended dominants”, as they form a circle of fifths (or of fourths, depending on which side you are looking at). The concept is simple, they are only dominant. We can improvise on them using the Mixolydian mode of each dominant, or the other approaches that we will study later on (later topics). Of course, this improvisation is not always easy, since these passages can be very fast, which would make the solo difficult. That is why it is important to train a lot on this topic, after all secondary dominants appear a lot in the harmoniously rich styles (jazz, bossa nova, mpb, etc.).
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