15.27Another powerful chromatic line: 5 → ♯5 → 6

Try:

C → E7 → Am

We already know:

G → G♯ → A

Scale-degree idea in C:

5 → ♯5 → 6

This is secondary-dominant voice leading.

Borrowed iv gives:

6 → ♭6 → 5

So now you have two opposite chromatic lines:

G → G♯ → A

and:

A → A♭ → G

These are extremely useful arranging devices.