Triads: Three Notes, Four Moods
Stack two thirds on any note and you have a chord. Which two thirds you stack — big or small, in which order — decides whether it smiles, sighs, grinds or floats.
Builds on: 3.1 Intervals: Naming the Distance
Harmony starts with three
Play C, then E, then G one after another and you have a melody. Play them together and you have a chord — three or more notes sounding at once. The simplest chord, and by far the most common, is the triad: three notes built by stacking thirds. Start on a note (the root), go up a third to the third, then up another third to the fifth. C–E–G is a triad: root C, third E, fifth G. Every pop song, every hymn and most of Bach can be reduced to a sequence of triads.
Big third, small third
Unit 3 taught you that a third comes in two sizes: major (4 semitones) and minor (3 semitones). A triad stacks two of them, so there are exactly four combinations — and each has a name and a personality:
- Major triad — major third, then minor third (4 + 3). C–E–G. Bright, settled, the sound of a sunny chorus. Symbol: just the root letter,
C. - Minor triad — minor third, then major third (3 + 4). C–E♭–G. Darker, wistful. Symbol:
Cm(orC−, or a lower-case numeral). - Diminished triad — two minor thirds (3 + 3). C–E♭–G♭. Tense and unstable; the fifth is squeezed down to a tritone. Symbol:
CdimorC°. - Augmented triad — two major thirds (4 + 4). C–E–G♯. Dreamy, unresolved, slightly alien; it splits the octave into three equal parts. Symbol:
CaugorC+.
Notice that major and minor share the same outer notes — a perfect fifth, C to G — and differ only in the middle. Move the third down one semitone and the whole mood turns from bright to sad. That single note is why the third is called the chord’s colour note: the fifth gives the chord its frame, the third fills it in.
A lead sheet writes chords as symbols above the melody: C, Am, F, G. The capital letter is the root; what follows is the flavour. Nothing after the letter means major. m means minor. Later lessons add 7, maj7 and friends — all of them are these four triads with extra notes on top.
Same notes, different order: inversions
A chord does not have to have its root at the bottom. Take C–E–G and move the C up an octave: E–G–C. Same three notes, same chord, but now the third is lowest — this is the first inversion. Move the E up too and you get G–C–E, the second inversion, with the fifth in the bass. Inversions matter for two reasons: they let a pianist move between chords without leaping, and they change how solid a chord feels — root position is the most grounded, second inversion the most floating. The chord’s name never changes; only its posture does.
Play a major triad and hold it. Now lower only the middle note by one key: minor. Now lower the top note as well: diminished. Go back to major and raise the top note instead: augmented. Four chords, two moving notes — the whole vocabulary of triads fits in one hand.
In the lab, build all four on any root, flip them through their inversions, and hear each one arpeggiated and then together. Watch the tower of thirds: the numbers between the notes are the recipe.
🎼 Lab — Chord Builder
Pick a root and a flavour; the tower shows the two stacked thirds with their semitone counts, the staff shows the chord, the piano shows the keys.
- Start on C major, switch to minor: only the middle note moves, and the mood flips.
- Compare diminished and augmented — both are symmetrical stacks, one squeezed, one stretched.
- Cycle the inversions and watch the same notes climb the staff without changing their name.
Work it by hand — 0 / 2
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±2% count.