12.6Secondary dominants in C major

The diatonic chords in C:

C Dm Em F G Am B°

Now give most of them their own dominant.

To Dm / ii

A7 → Dm

so:

A7 = V7/ii

To Em / iii

B7 → Em

so:

B7 = V7/iii

To F / IV

C7 → F

so:

C7 = V7/IV

To G / V

D7 → G

so:

D7 = V7/V

To Am / vi

E7 → Am

so:

E7 = V7/vi

We normally don't talk about a secondary dominant of vii° nearly as often because that diminished chord is unstable and not a typical tonicized destination.