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In music, an 'augmented triad' is a triad consisting of two major thirds. (If an octave from the root is added, the resulting chord contains also a diminished fourth – the enharmonic equivalent of a major third.) It is called an augmented triad because the two major thirds together make up an augmented fifth. However, in equal temperament the diminished fourth is enharmonically equivalent to an ordinary major third, and the augmented fifth to a minor sixth.
An example of an augmented triad is A♭ C E, where the interval from E to A♭ is a diminished fourth, and from A♭ to E an augmented fifth.
In just intonation, the interval between two major thirds and an octave, 2/(5/4)², is 32/25, which is flatter by a septimal kleisma of size 225/224 than the septimal major third with ratio 9/7. While septimal meantone temperament tempers out the septimal kleisma, some other temperaments, for example miracle temperament, do so also, and in all of these temperaments the augmented triad may be identified with a circle of two major and one septimal major thirds, making up an octave.

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The Augmented Triad in Tonal Music

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The Augmented Triad in Tonal Music

The augmented triad differs from the other kinds of triad (the Major triad, the minor triad, and the diminished triad) in that it does not naturally arise from the diatonic collections of tonal music (the Major scale and the minor scale). Although it could be conceptualized as a triad built on the third degree of a harmonic minor scale or melodic minor scale, it virtually never occurs in this way (since any chord on the third degree is itself rare, usually being a new tonic).
This makes the augmented triad a special chord that touches on the atonal. Its uses to 'suspend' tonality are famous, for example in Liszt's Faust Symphony and in Wagner's Sigfried Idyll. However, the augmented triad occurs in tonal music, with a perfectly tonal meaning, since at least Beethoven. It results diatonically in minor mode from a dominant chord where the fifth (the second degree) is replaced by the third degree, as an anticipation of the resolution chord. Beethoven's 9th symphony features such a chord at key moments in the slow movement. Brahms's Tragic Overture also features the chord prominently (A-C#-F), in alternation with the regular dominant (A-C#-E). In this example one can also see other aspect of the appeal of the chord to composers: it is a 'conflation' of the fifth degree and the third degree, the usual contrasting keys of a piece in the minor mode.
With the lead of Schubert (in his Wanderer Fantasy), Romantic composers started organizing many pieces by descending major thirds, which can be seen as a large-scale application of the augmented triad (although it probably arose from other lines of development not necessarily connected to the augmented triad). This kind of organization is common, in addition to Schubert, in the music of Liszt, Rimsky-Korsakov, and Wagner, among others.

See also

★ Augmented chord

★ Diminished chord

★ Triad