Triads, and the four possible qualities
Three notes stacked in thirds, and why only four combinations exist, all of them determined by the intervals, not by anyone's choice.
A chord is several notes sounding together. The triad is the smallest chord that still has an identity: three notes stacked in thirds.
The stacking
Take a note, add a third above it, add another third above that. The three notes get names from the place they occupy:
- Root — the note that names the chord
- Third — decides whether the chord is major or minor
- Fifth — holds the chord up; when altered, it changes everything
Only four exist
Two stacked thirds, each major or minor. Four combinations, and that is literally all there is:
Major — major third below, minor above:
— C, E, G. and then . The fifth comes out perfect.
Minor — minor third below, major above:
— C, E♭, G. and then . The fifth is still perfect.
Diminished — two minor thirds:
— C, E♭, G♭. The fifth shrinks to , and the chord loses its footing.
Augmented — two major thirds:
— C, E, G♯. The fifth stretches to plus a semitone, and the chord hangs suspended, with no obvious direction.
What the third does
Change one note, the third, and the chord changes character entirely. Major and minor differ by a semitone, and that semitone is the difference between music that sounds open and music that sounds closed.
The fifth, by contrast, is hardly noticed while it is perfect. It only draws attention when it is altered, and then it dominates the whole chord.
Inversions
The same three notes, stacked in a different order, are still the same chord. The bass changes, and with it the weight:
- Root position: C, E, G —
- First inversion: E in the bass —
- Second inversion: G in the bass —
Chord symbols write this with a slash: the letter before the slash is the chord, the letter after it is the bass note. In it is still a C chord, merely resting on E.
Where they appear
Apply the stacking to each degree of a major scale and the four qualities distribute themselves: three major, three minor and one diminished:
Nobody chose that distribution. It is what is left when you stack thirds using only the notes of the scale.
What to take from this
Two stacked thirds, four possible results. The third decides the character, the fifth decides the stability, and the inversion decides the weight, without changing the chord's identity. Everything else in harmony is built on top of this.