Augmented Triad - in Tonal Music

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 in a diatonic 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 rarity 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 Siegfried Idyll. However, the augmented triad occurs in tonal music, with a perfectly tonal meaning, since at least Bach (see the first chord in the opening chorus to his cantata Ach Gott, vom Himmel sieh darein, BWV 2) and Haydn (see, for example, the Trio from Haydn's String Quartet Op. 54/2). 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, it is found in music of Liszt, Rimsky-Korsakov, Louis Vierne and Wagner, among others.