The idea
Every major or minor diatonic chord can be preceded by its own dominant, as if for an instant it were a tonic (tonicization). It is written V/x (“five of x”).
In C major
- V7/ii = A7 → Dm (C# is the leading tone of D)
- V7/iii = B7 → Em (D# is the leading tone of E)
- V7/IV = C7 → F (Bb is the seventh that falls to A)
- V7/V = D7 → G (F# is the leading tone of G)
- V7/vi = E7 → Am (G# is the leading tone of A)
The vii° (B°) is not tonicized, because a diminished chord cannot be a tonic.
The related ii
Every secondary dominant can have its own ii: Em7 – A7 – Dm7 (ii–V of ii), F#m7b5 – B7 – Em (minor ii–V of iii). Berklee courses call it the “related II.”
Extended dominants
A chain of dominants that resolve into one another without returning immediately to the diatonic: E7 – A7 – D7 – G7 – C. Each chord is the V of the next; the feeling is of a continuous push toward the tonic.
Deceptive resolutions
A secondary dominant can resolve to a chord different from the expected one: A7 → Bb instead of Dm, E7 → F instead of Am. It is a powerful tool for pre-choruses.
Voice leading
The chromatic note (C#, F#, G#…) behaves as a leading tone and rises; the seventh falls. Watch the melody: if it sings C over A7 it clashes with the C#.
Real example
“Yesterday” by the Beatles (1965, in F) opens with F – Em7 – A7 – Dm: Em7–A7 is a ii–V of vi, with A7 as the secondary dominant. Transposed to C: C – Bm7 – E7 – Am. “Sweet Georgia Brown” (1925) is instead the classic case of chained extended dominants.