15.7The voice-leading formula

In any major key:

IV contains scale degree 6

borrowed iv contains ♭6

then tonic contains 5

So the inner line is:

6 → ♭6 → 5

In C:

A → A♭ → G

In G:

E → E♭ → D

In D:

B → B♭ → A

This is an important reusable chromatic line.