9.1The central harmonic contrast

A simplified conceptual skeleton is:

C major area

then:

D7

then back toward:

Dm7 → G7 → C

The important unusual chord is:

D7 in the key of C

D7 contains:

D F♯ A C

The F♯ is not diatonic to C major.

So immediately, the chord has a bright chromatic color.