Section 21.1 Augmented Sixth Chords
Augmented sixth chords are a special class of pre-dominant chords with notes that approach the dominant (\(\hat{5}\) ) from a half-step below (♯\(\hat{4}\) ) and from a half-step above (♭\(\hat{6}\) ) simultaneously.
In minor, ♭\(\hat{6}\) doesn't require a flat because \(\hat{6}\) is lowered in the minor key signature.