Folks, got this from the net when studying some chord theory:
The only other triad which appears to have full mutual consonance is the augmented triad (1 - 3 - sharp5). On a piano keyboard this triad appears to contain two major thirds and a minor sixth - all of which are consonant, but in actuality the interval between the top and bottom tones is not the consonant minor sixth but the dissonant augmented fifth.
The augmented fifth and the minor sixth may be represented by the same number of semitones in 12-tone equal temperament, but this does not mean that the aural effect of these two intervals is the same.
This may seem paradoxical and somewhat pedantic, but it is not - neither in a theoretical nor an empirical sense. The augmented fifth is a disturbing interval, even when expressed in equal temperament.
Can the experts explain how can the same 8 semitone interval be consonant and dissonant at the same time