13.52Add extensions to secondary dominants

Our Module 12 progression:

Cmaj7 → E7 → Am7 → A7 → Dm7 → G7 → Cmaj7

can become:

Cmaj9 → E7♭9 → Am9 → A13 → Dm9 → G13 → Cmaj9

That looks advanced.

But underneath it is still:

I → V/vi → vi → V/ii → ii → V → I

Same route.

More color.