An old friend: the augmented sixth · Tritone substitution · Music theory
Jazz did not invent this sonority. Classical music had been using augmented sixth chords since the 18th century, resolving to the dominant with the bass…
An old friend: the augmented sixth
Jazz did not invent this sonority. Classical music had been using augmented sixth chords since the 18th century, resolving to the dominant with the bass falling a half step. One of them, the so-called German sixth, is exactly the same sound as the substitute of V⁷/V.
In C major, A♭, C, E♭, F♯ resolves to G; A♭7 has those notes except for the spelling: G♭ instead of F♯. The spelling changes according to where the voice goes: F♯ rises to G, G♭ falls to F. So a single chord can open the door to another key, without the ear knowing until the resolution.