21.4: Lead-Sheet Analysis of Augmented Sixth Chords
When using lead-sheet symbols for augmented sixth chords, we will treat them as major-minor seventh chords built on ♭6^ , since that matches the sonic quality of the augmented sixth chords and is similar to how they occur and are spelled in jazz and popular music. The chords in parentheses are enharmonic respellings of the three augmented sixth chord types.
In the example above, we see a disagreement between “classical” analysis and lead-sheet analysis. The fundamental concept of lead-sheet analysis is to show root, quality, and inversion. The “classical” spelling and the classical analysis of augmented sixth chords show the direction of resolution (with the chromatically raised notes), which necessitate the need for original labels, since we don't have a Roman numeral or a lead-sheet symbol that would communicate a chord containing a diminished third above the root (that is then inverted!).
Therefore, when you are asked to provide lead-sheet symbols for augmented sixth chords, analyze each one as if it is a dominant 7th chord with a misspelled 7th (AA♭–CC–EE♭–FF♯ instead of AA♭–CC–EE♭–GG♭).