The four qualities
A triad is made of a root, third and fifth stacked in thirds.
- Major: major third + minor third (C E G). Symbol C.
- Minor: minor third + major third (A C E). Symbol Am.
- Diminished: two minor thirds (B D F). Symbol B° or Bdim.
- Augmented: two major thirds (C E G#). Symbol C+ or Caug.
Suspended triads (Csus2 = C D G, Csus4 = C F G) replace the third: they are neither major nor minor.
Diatonic triads
In C major: C Dm Em F G Am B° → I ii iii IV V vi vii°.
In natural A minor: Am B° C Dm Em F G → i ii° III iv v VI VII.
With the harmonic minor the V becomes E (major), the vii° becomes G#° and the III becomes C+ (C E G#).
Root position and inversions
With the root in the bass the chord is in root position; with the third in the bass it is in first inversion (C/E); with the fifth in the bass it is in second inversion (C/G). The quality doesn't change; stability and bass line do.
Why it matters
Triads are the minimal grammar: with the six major and minor triads of a key you write the great majority of pop. The diminished triad contains a tritone and is unstable; the augmented is symmetrical and ambiguous, useful as a passing chord (C – C+ – Am or C – C+ – F).
Real example
“Let It Be” by the Beatles (1970) is in C major and uses almost only four diatonic triads: C G Am F (I V vi IV). It is proof that a few well-ordered triads are enough for a complete song.