Triads: Major, Minor, Diminished, Augmented
Stacking thirds to build the four triad qualities, and how to recognize each by ear and by interval content.
Stack two thirds on top of a root and you get a triad — the basic chord of tonal music. What kind of triad you get depends entirely on the size of those two thirds.
Major third + minor third = major triad (e.g. C-E-G): bright, stable, the sound most people mean by "happy." Minor third + major third = minor triad (C-Eb-G): darker, but just as stable — the root and fifth are a plain perfect fifth in both cases, so major and minor triads feel equally at rest.
Minor third + minor third = diminished triad (C-Eb-Gb): both thirds minor, and the outer interval shrinks to a tritone — unstable, wanting to resolve. Major third + major third = augmented triad (C-E-G#): both thirds major, the outer interval stretches to an augmented fifth — also unstable, but in a hazier, more ambiguous way than diminished.
In the exercise below, three notes on the piano light up as a triad. Play it, listen to how stable or restless it feels, and match that feeling to the quality — major/minor triads settle, diminished/augmented triads pull toward something else.