10.23Hear Dm7 versus D7

Play:

Dm7 → G7 → Cmaj7

Then:

D7 → G7 → Cmaj7

The D7 introduces F♯, which points toward G.

It becomes a dominant of the dominant.

We will call that a secondary dominant.

So:

Dm7 = ii in C

D7 = V7 of G

Very different job.