A secondary dominant is a V chord, or extension thereof (V7, V9, Vll, V13, V7sus4, etc.), which is not diatonic to the key, although it must have a diatonic root. For example, in the key of C major, secondary dominants would be C7, D7, E7, F7, A7 and B7 but not G7 as this chord is wholly diatonic to the key.
Secondary dominants can be identified by the chord to which they lead. For example, in the key of C major, B7 is a secondary dominant which resolves to Em7. In C major, Em7 is the iii chord, thus allowing B7 to be referred to as the V7 of iii, written as V7/iii.
Another example could be that of E7. This would resolve to Am7 in the key of C major. Am is the vi chord of C major. and so E7 could be referred to as V7/vi.
The following are the secondary dominants in the key of C major. showing the chords to which they resolve. Remember that the V chord, G7, is not a secondary dominant as it is wholly diatonic to the key. Also note that F7, whilst still being a secondary dominant, does not resolve to a diatonic chord, hence the designation V7/bvii.
Secondary dominants are a convenient way of modulating to other key centres. Consider the following example, where the key centre moves from F major to G major:
| || | F 7 |
| | Dm7 | | | Am7 | | | D7 | | | G7 |
|| | |
| F major: | I7 |
vim7 | iiim7 | V7/ii | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| G major: | iim7 | V7 | I7 |
||||||||
D7 here is functioning as a secondary dominant of the initial key of F major, and as the V chord of G major. The common chord of Am7 (iii of F, ii of G) is also instrumental in setting up smooth use of the secondary dominant (root note is moving from A to D, up an interval of a perfect fourth).
Improvising over secondary dominants poses special problems as the chords themselves are not diatonic to the key. A number of chord/scale options are available:
1. Each secondary dominant can be treated as a normal V7 chord of a major key, and the mixolydian mode employed, e.g.:
| || | E7 | | | A7 | | | D7 | | | G7 | | | C7 | | | etc... |
| E mixolydian | A mixolydian | D mixolydian | G mixolydian | C mixolydian |
2. If the root of the secondary dominant is the 3rd or 7th degree of the key (e.g., E or B in the key of C major), the Phrygian major may be used (the V mode of the harmonic minor - more about this elsewhere).
For example, in the key of C nalor, the correct mode to use over B7 (V7/iii) is B phrygian major (the V mode of E harmonic minor - B7 resolves to Em7, c.f. V7 to im7 in a minor key). Similarly, for E7 (V7/vi), E phrygian major could be used (V mode of A harmonic minor).
The reason that the phrygian major works so veil is because the chordal tones of the secondary dominant are included in the harmonic minor, but passing tones are closer Co the original key than if you use the mixolydian mode. Because the passing tones are closer to the original key centre, transition from the original key to the secondary dominant is smoother. Examine the following situation where the secondary dominant E7 (V7/vi) occurs in the key of C major:
It is therefore preferable to use the phrygian major over the secondary dominant.
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