Augmented fifth

The augmented fifth 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 unstable seventh degree (or leading tone) 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).

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 other contexts, however, the same eight-semitone interval will simply be heard (and notated) as its consonant enharmonic equivalent, the minor sixth.