§ Partimento Chord Progression Theory

• consider the ones in the key of C minor.
• The chord that corresponds to Ab is given as D F# C.
• Luisa's insight: Consider the 5th, which is Gm. It's 7th scale degree is F#.
• Luisa's instruction: Think of this the other way round: Note that we see an F#, which we recognize as the 7th scale degree of the Gminor scale.
• Now, what is Ab D F# C in Gm? That's the question we try to answer.
• This is a french sixth or augmented sixth
• We start with Ab, which is the sixth in the scale degree.
• We throw in the third, which is C.
• We then add build the augmented sixth interval with Ab as the lower pitch, giving us F#.
• Now, it is popular wisdom that a sixth comes with a fourth (the D wrt Ab), with the idea that we resolve the 1 3 4 6 → 1 3 5.
• However, the actual resolution in the book is more interesting: We have Ab D F# C → G D G B, where the D does not "actually move", the F# → G, and the C → B, and Ab → G. Note that this is pretty genius, because it only moves by a single semitone.
• The Ab ⇒ G changes the chord, while the other two notes resolve.