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6.9.5 Secondary Dominants: Dominant Chords of Chords Other Than Tonics
Discussions of secondary dominants often get complicated and mystical. No need; it’s completely straightforward.
A secondary dominant is a dominant chord of a chord other than the tonic.
You can create secondary dominants and see them easily on any chord map. In Figure 69 below, the chord map shows that, in the key of C major/A minor, there are always two dominant chords, the G7 and E7 chords. Each has a dominant function over its tonic chord, C major and A minor, respectively.
To create a secondary dominant, simply replace some other chord in the circular harmonic scale with its namesake dominant 7th chord. That's all there is to it. For example, if you replace the Am chord with an A7 chord, that A7 chord becomes a secondary dominant because it now has a "dominant" function with respect to the next fifth-down chord (Dm, or any variant), just as G7 has a dominant function with respect to C major. Similarly, if you replace Dm with D7, the D7 chord becomes a secondary dominant with respect to the next fifth-down chord (G7, or any variant).