When the six chords in the key are not enough, this is the cheapest move with the biggest payoff. Swap one chord and the progression gains depth.
Chapter 12 said that G (tension) wants to resolve to C (home). That pulling force can be built towards chords other than home too.
Put E7 in front of Am
Heading for Am in the key of C, use E7 instead of Em just before it. E7 contains G♯, a note from outside the key, and that note pulls hard towards the A in Am.
- Em, staying in key
C | Em | Am | G
- E7, one note from outside
C | E7 | Am | G
The E7 version arrives at Am with much more conviction. That is what sounds dramatic. One borrowed note — G♯ — does all of it.
A tension chord borrowed temporarily to aim at a chord other than home. Aiming at Am, the "dominant of Am" is E7.
On the circle of fifths it is right next door
Below is the circle of fifths: major keys outside, their related minors inside, moving up a fifth each step clockwise. C and Am share a position at twelve o'clock, and E — which pulls towards Am — sits a little clockwise from there. Press any of them to hear the chord.
The circle shows at a glance which chords are closely related. Clockwise neighbours pull; anticlockwise neighbours open out. The next chapter uses the same diagram for modulation.
Generate over a progression containing E7. The note from outside the key widens the melodic options too.
Generate over C, E7, Am, GAny chord can have a chord placed before it that pulls towards it. One of these makes a progression sound suddenly grown-up.