Augmented

Take any 2 adjacent notes in an augmented triad (assuming going in order of 1-3-#5), and they have an interval of a major 3rd, which is consonant. Interestingly, it's the addition of the minor 6th interval that creates the tension that an augmented chord is known for. But because augmented chords are symmetrical, that interval appears 3 times

Theory

the augmented triad is symmetrical. It divides the octave into three equal parts. That symmetry is acoustically "unnatural" — the harmonic series is not symmetrical, it's a hierarchy with the perfect fifth at the top. A chord that perfectly balances three equal intervals has no internal gravity, no implied root, no direction. Your ear can't locate a tonal centre within it. That rootlessness is the dissonance, or at least a major source of it.

Compare this to a major triad: the 4:5:6 ratio creates a clear acoustic hierarchy with a strong root. The augmented triad has no such simple integer ratio — it's built on the irrational number 2^(1/3), which has no clean relationship to the harmonic series

Take any augmented chord and lower any one of the chord tones by a half-step, and you end up with a major chord. Now, take that same augmented chord but raise a chord tone by a half-step and you end up with a minor chord.

Example use:

• Fixing a Hole - intro, used as a passing chord (the second chord of the harpsichord intro)