14.4What is G7 theoretically?

In F major, G7 can be heard as:

V7/V

because G7 can lead to C7,

and C7 is V of F.

So:

G7 → C7 → F

would be a clear secondary-dominant chain.

But if G7 sits there for a while, it also becomes a color area of its own.

Exactly the lesson from “Take the A Train.”