Music Theory for the 21st-Century Classroom

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.