Augmented triad
Two major thirds stacked on top of each other. It divides the octave evenly and therefore has no unambiguous root.
What it means
The augmented triad consists of two major thirds stacked on top of each other; between the root and the fifth lies an augmented fifth. In its basic form over C it consists of C, E, and G-sharp. The completion to the octave — from G-sharp to the upper C — is enharmonically again a major third, so that the chord divides the octave into three equal parts.
Musical use
From this symmetry follows its most important property: all inversions sound the same, and no note stands out as the root. There are only four different augmented triads; all others are enharmonic spellings of these four. Which note counts as the root in a given case results from the spelling and from the context.
This ambiguity makes the sonority flexible. Each of its notes can lead, by a semitone step, into a neighbouring major or minor sonority, and this enables surprising turns and remote modulations. In functional harmony it mostly appears as a dominant with a raised fifth, whose raised note strives, as an additional leading tone, toward the third of the tonic, or as a passing sonority between a triad and its six-three position. As a scale-native chord it stands on the third degree of the harmonic minor scale.
Its notes are all contained in the whole-tone scale, with which it shares the principle of even spacing. In the music around 1900 it therefore often appears as a timbre without any compulsion to resolve.
In the Romantic era it is gladly used as a passing sonority, for instance when the fifth of a major triad rises chromatically to the sixth.
Other names
Augmented chord follows the English term; in chord symbols “aug” or a plus sign stands. Since the augmented triad divides the octave into three major thirds, there are only four sonically different augmented triads, and each can be reinterpreted enharmonically.
Performance practice
Determine the root from the spelling and lead the augmented fifth onward upward. Intonate it rather high, so that the pull becomes audible. And where the sonority stands as a colour, give it time — its effect lies in floating, not in pressing forward.