We’ve seen that the V – I progression is the strongest chord progression in our music.
And by building on that, we can extend that chord progression by preceding the V chord with IT’S V chord: the V of V.
In the original key that comes down to being the II chord.
In the original key the II chord is always minor.
We’ve seen this in the key of G:
the II – V – I progression is Am7 – D7 – G.
But we can make that progression even stronger by turning that Am7 into an A7 chord.
Any V chord wants to resolve to any I chord, meaning:
an A wants to go to a D
an Am wants to go to a D
an Am wants to go to a Dm
an A7#9b13 wants to go to an Dm7
Some of these progressions sound stronger than others.
The more tension a V chord has the more it wants to resolve. And because we know that the A7 chord has a lot of tension (the tritone interval !) it reaaally wants to resolve to the D chord.
We can use this in our II – V – I progression.
Instead of using an Am7, we’re gonna use an A7 chord, making the progression
A7 – D7 – G7 or II7 – V7 – I .