In classical music from Western culture, an augmented fifth is an interval produced by widening a perfect fifth by a chromatic semitone. For instance, the interval from C to G is a perfect fifth, seven semitones wide, and both the intervals from C to G, and from C to G are augmented fifths, spanning eight semitones. Being augmented, it is considered a dissonant interval. Its inversion is the diminished fourth, and its enharmonic equivalent is the minor sixth. 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 to match that of the unstable seventh degree of the major mode. A consequence of this was that the interval between the minor mode's already lowered third degree and the newly raised seventh degree, previously a perfect fifth, had now been "augmented" by a semitone. Another result of this practice was the appearance of the first augmented triads, built on the same degree, in place of the naturally occurringmajor chord. As music became increasingly chromatic, the augmented fifth was used with correspondingly greater freedom and also became a common component of jazz chords. Near the end of the nineteenth century the augmented fifth became commonly used in a dominant chord. This would create an augmented dominant chord. The augmented fifth of the chord would then act as a leading tone to the third of the next chord. This augmented V chord would never precede a minortonic chord since the augmented fifth of the dominant chord is identical to the third of the tonic chord. In an equal tempered tuning, an augmented fifth is equal to eight semitones, a ratio of 22/3:1, or 800 cents. The 25:16 just augmented fifth arises in the C harmonic minor scale between E and B. 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 as its consonantenharmonic equivalent, the minor sixth.
