Augmented fifth
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An augmented fifth is one of three musical intervals that span five diatonic scale degrees. The prefix 'augmented' identifies it as being the largest of the three intervals; the others being the perfect fifth and diminished fifth, which are one and two chromatic semitones smaller, respectively. Its inversion is the diminished fourth, and its enharmonic equivalent in equal temperament is the minor sixth.
The augmented fifth has no 'natural' diatonic occurrence, and only began to make an appearance, at the beginning of the common practice period of music, as a consequence of composers seeking to strengthen the normally weak seventh degree when composing music in minor modes.
This was achieved by chromatically raising the seventh degree (or subtonic) to match that of the more dynamic seventh degree (or leading note) of the major mode (an increasingly widespread practice that led to the creation of a modified version of the minor scale known as the harmonic minor scale).
A consequence of this was that the interval between the minor mode's already lowered third degree (mediant) and the newly raised seventh degree (leading note), previously a perfect fifth, had now been 'augmented' by a chromatic semitone. This resulted in the first diatonic use of the augmented fifth interval.
Another result of this practice was the appearance of the first augmented triads, built on the same (mediant) degree, in place of the naturally occurring major chord.
As music became increasingly chromatic, the augmented fifth was used with correspondingly greater freedom and also became a common component of jazz chords.
In an equal tempered tuning, an augmented fifth is equal to eight semitones, a ratio of 1:28/12 (approximately 1.587), or 800 cents.
The augmented fifth is a context-dependent dissonance. That is, when heard in certain contexts, such as that described above, the interval will sound dissonant. In other contexts, however, the same eight-semitone interval will simply be heard (and notated) as its consonant enharmonic equivalent, the minor sixth.
