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Secondary Dominant Chords, Tonicization, and Related II-7
Table of Contents
Secondary Dominants
Secondary dominant chords are dominant seventh chords that temporarily tonicize a diatonic chord without modulating to a new key. Tonicization is simply the process of making a chord that is not the tonic of the key briefly sound like the tonic. This is different than modulation, which is a shift into a new key for an extended period of time.
Each of the diatonic chords in a key has a V7 that strongly wants to resolve to the target chord. This creates a sense of more tension and resolution even when not on the V7-I of the key. Notice that some of the secondary dominants have the same root as a diatonic chord but have a different chord quality. One example of this is V7/ii (A7 to D-7). Here, the secondary dominant's root is the VI of the key, but the chord is a dominant seventh instead of a minor seventh.
Related II-7 Chords
In the article on harmonic function, we described the subdominant --- dominant --- tonic progression as one of the fundamental units of tonal jazz. This takes us from a state of some dissonance to maximum dissonance, to consonance. With related ii7 chords, we can create this tonal functional sound briefly for a chord that is not the tonic. Each secondary dominant chord also has a related ii-7 chord. This allows us to briefly extend the tonicization while returning quickly to the home key.
The image at the top of this article shows the secondary dominants and related ii-7 chords for diatonic chords in C major and C natural minor (Aeolian). By using this as an example you should be able to infer the secondary dominants for other keys and other scales.