12.18Secondary dominants versus diatonic dominants

In C major:

G7

is the primary dominant.

Why?

Because G is V of C.

But:

A7, B7, C7, D7, E7

are secondary dominants when used to tonicize diatonic chords of C.

So:

• G7 = V7
• D7 = V7/V
• E7 = V7/vi
• A7 = V7/ii
• B7 = V7/iii
• C7 = V7/IV