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An augmented sixth is one of three musical intervals that span six diatonic scale degrees. The prefix 'augmented' identifies it as being the largest of the three intervals; the others being the major sixth and minor sixth, which are one and two semitones smaller, respectively. Its inversion is the diminished third, and its enharmonic equivalent is the minor seventh.

In the tuning system known as equal temperament the augmented sixth is equal to ten semitones and is a dissonant interval.

The augmented sixth is relatively rare. Its most common occurrence is built on the lowered submediant of the prevailing key, in which position the interval assumes a natural tendency to resolve by expanding to an octave built on the dominant tonal degree. In its most common and expected resolution, the lower note of the interval moves downwards by a minor second to the dominant while the upper note, being chromatically inflected, is heard as the leading note of the dominant key, rising naturally by a minor second. It is the strong tendency to resolve in this way that properly identifies this interval as being an augmented sixth rather than its more common enharmonic equivalent: the minor seventh, which has a tendency to resolve inwardly.

As the augmented sixth is correctly named only in certain specific contexts, the notational distinction between it and the identical sounding minor seventh is often ignored. Regardless of the true diatonic context, many writers choose in favour of the more familiar minor seventh - especially in chord notation, in which chords containing it are always labelled seventh chords.

The augmented sixth interval in combination with certain other intervals forms the group of chords known collectively as augmented sixth chords.

• augmented sixth chord
• list of meantone intervals