Augmented sixth

In music, an augmented sixth (Play^ⓘ), ^A6, is an interval produced by widening a major sixth by a chromatic semitone. For instance, the interval from C to A is a major sixth, nine semitones wide in 12 TET, and both the intervals from C♭ to A, and from C to A♯ are augmented sixths, spanning ten semitones (in 12 TET).

Being augmented, it is nominally considered a dissonant interval, even though it renders a perceptibly consonant harmonic seventh in some tuning systems:

In septimal meantone temperament, an augmented sixth is specifically assigned to the harmonic seventh (a consonant just interval of 7:4) and very nearly so in quarter comma meantone and 31 TET. In 12 TET, the augmented sixth is equal to ten semitones, and is both nominally and audibly dissonant.

An augmented sixth (^A6) is enharmonicly equivalent to a minor seventh (m7). An inverted ^A6 is a diminished third.