Augmented triad
The augmented triad is a triad comprising a root, a major third, and an augmented fifth formed by stacking another major third.
In temperaments
In augmented temperament, the ~25/16 augmented fifth is enharmonically equivalent to a minor sixth (~8/5), the difference of 128/125 being tempered out. Since the resulting internal intervals are ~5/4, ~5/4, and ~8/5, the augmented triad in augmented temperament is a 5-odd-limit essentially tempered chord:
- (Augmented) 1 – 5/4 – 8/5
The gap remaining to close the octave is another identical fifth (~5/4), giving rise to the temperament's name after this chord.
If 225/224 is tempered out, as in marvel and septimal meantone, the ~25/16 augmented fifth is tempered together with ~14/9. The resulting intervals are ~5/4, ~5/4, and ~14/9, making the augmented triad a 9-odd-limit essentially tempered chord (→ Marvel chords).
- (Marvel) 1 – 5/4 – 14/9
In that case, the gap remaining to close the octave is ~9/7, a septimal major third.
Since 12edo is a good tuning of augmented temperament and supports septimal meantone, and the historically prevalent quarter-comma meantone is a good tuning of septimal meantone, either or both of these essentially-tempered interpretations may be relevant for augmented chords found in common-practice and contemporary music.
In just intonation
In the 7-limit:
- 36:45:56 is the simplest JI tuning of the 7-limit marvel augmented chord.
In the 5-limit:
- 16:20:25 is found on ♭II+ (16⁄15), ♭III+ (6⁄5) ♭VI+ (8⁄5), and ♭VII+ (9⁄5) in the duodene. Its rotations, 20:25:32 and 25:32:40, represent chords that would be enharmonically equivalent to itself in augmented temperament, substituting a minor sixth and a diminished fourth successively in place of the augmented fifth and major third.
In the 3-limit:
- 4096:5184:6561 may be considered a 3-limit approximation of 16:20:25.