Triads
Root, third, fifth. Four types, distinguished entirely by which kinds of third are stacked.
A triad is a three-note chord built by stacking thirds. Take a note, add the note a third above, then another third above that.
The three notes have names:
- Root — the note the chord is named after
- Third — a third above the root; decides major or minor
- Fifth — a fifth above the root; usually the stable outer note
- Gthird
- Emajor
- Croot
Four types
Which kind of third you stack determines everything.
| Triad | Lower third | Upper third | Root to fifth | Example |
|---|---|---|---|---|
| Major | major | minor | perfect fifth | C E G |
| Minor | minor | major | perfect fifth | C E♭ G |
| Diminished | minor | minor | diminished fifth | C E♭ G♭ |
| Augmented | major | major | augmented fifth | C E G♯ |
Read that table by rows and a pattern appears: major and minor use the same two thirds in opposite orders. Diminished and augmented double up on one kind.
Major and minor are the ones you will use
Diminished and augmented triads are real and useful, but they are unstable by design — they contain a dissonant interval and want to move somewhere. Major and minor triads can sit still, so they carry almost all the weight in most music.
The difference between them is one note moving by one semitone, which is worth a page of its own.
Where the stability comes from
Both major and minor triads contain a perfect fifth between root and fifth. That interval is acoustically the most stable after the octave, and it is what lets the chord sound resolved.
Diminished and augmented triads alter that fifth. Take away the perfect fifth and the chord loses its floor — which is exactly why they sound like they are going somewhere.
Triads in a key
Build a triad on each degree of a major scale using only notes from that scale and you get a fixed pattern of qualities:
I major, ii minor, iii minor, IV major, V major, vi minor, vii° diminished
That pattern is identical in every major key, which is why chord progressions transpose so cleanly. See diatonic chords.