An augmented sixth is an interval between two notes in music. It is equivalent to 10 semitones between the notes, and is enharmonically equivalent to a minor seventh. What makes an augmented sixth what it is instead of a minor seventh is how the notes are spelled.

For instance, the interval from C to B flat is a minor seventh whereas the interval from C to A sharp is an augmented sixth.

This distinction is used to denote functional differences in music, it shows the difference between a dominant seventh chord and an augmented sixth chord.