Music theory / secondary dominants
Secondary dominants: borrowed V chords that make a phrase stronger
A secondary dominant is a V chord borrowed from a key other than the tonic. It tonicizes the chord that follows for one beat, giving that chord the pull of a real tonic. In C major, V/V is D major (not D minor); V/vi is E major (not E minor). The trick is simple once you see it.
Every chord in a key has its own dominant, waiting one fifth above. Borrow that dominant for a beat and the target chord feels temporarily like a tonic. This is the whole mechanism.
V/V, the most common secondary dominant
In C major, the V chord is G. The dominant of G is D major. In diatonic C major, the ii chord is D minor. Replace D minor with D major (raise the F to F#), and you have V/V. A progression like C-D-G-C uses V/V to strengthen the arrival on G.
V/vi, the darker one
In C major, the vi chord is A minor. The dominant of A minor is E major. The diatonic iii chord is E minor. Swap in E major and you have V/vi. Try C-E-Am: the E major chord makes A minor feel like a proper resolution, not just a passing chord.
V/ii, V/iii, V/IV
Every diatonic chord that can be a target (ii, iii, IV, V, vi) has its own borrowed dominant. V/ii in C major is A major. V/iii is B major. V/IV is C7 (yes, the tonic seventh chord functioning as V/IV; that is why C7 pulls to F). Vii diminished does not usually get a secondary dominant because it does not function as a target of tonicization.
Piston defines a secondary dominant as ‘the dominant of any diatonic chord other than the tonic’, notes that the secondary dominant is always a major triad or dominant seventh, and adds that the target chord must itself be capable of functioning as a temporary tonic (which is why vii° is rarely tonicized).
Piston, Harmony, 5th ed, ch. 16.
Hear one in every key
Open the chord progression generator, pick jazz preset, and generate. Most jazz-preset progressions include at least one secondary dominant, usually V/ii or V/V.
Where to hear them in real music
The bridge of most Beatles songs. Any Great American Songbook standard (Autumn Leaves, Fly Me to the Moon). The opening bars of most Bach chorales. Once your ear picks up the raised-third quality of a borrowed major triad, you cannot unhear them.
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