The Diatonic Triads

01What "diatonic" means

Before chords, the word. Diatonic means belonging to the key. If you're in C major, only the seven natural notes — C, D, E, F, G, A, B — are diatonic. The five black keys are not. They're chromatic.

The word predates piano by about 2,300 years. The Greeks used it to describe scales built mostly from whole steps — the through-line of tones — as opposed to chromatic or enharmonic genera that crammed in smaller intervals. We've kept the meaning: diatonic = in the key, no accidentals.

Once we know what's in the key, we can ask the only interesting question: what happens when we stack thirds on every step of it?

02Seven scale degrees, seven triads

A triad is built by stacking two thirds. You already know this from the chord qualities module: M3 + m3 = major, m3 + M3 = minor, m3 + m3 = diminished, M3 + M3 = augmented.

Now do it on every note of the C major scale, using only the white keys. Stack a third, stack another third. No black keys allowed — that's the diatonic constraint. Click each one.

The pattern: major, minor, minor, major, major, minor, diminished. No accidents — every quality is forced by which white-key intervals happen to land in that stack.

03Why the qualities are fixed

The major scale is not seven equally-spaced notes. It's a specific arrangement of whole steps (W) and half steps (H):

Two half steps — between E–F and between B–C — sit at fixed positions. Everything that follows is consequence.

Stack a third from any scale degree using only diatonic notes. Sometimes that third spans W+W (a major third, 2 whole steps). Sometimes it spans W+H (a minor third, 1½ steps). The half steps don't move — so the major and minor thirds land in a fixed pattern:

That's the whole derivation. No memorization. Just: where do the half steps land inside each third you stack.

04The formula

Music writes scale degrees in Roman numerals. Capital = major. Lowercase = minor. A small circle = diminished. In any major key, the pattern is:

Three things to lock in:

• I, IV, V are always major. The three majors. These three chords alone built half of every song from the 1950s.
• ii, iii, vi are always minor. The three minors.
• vii° is diminished. Rarely used as its own chord — it functions mostly as a tension-builder leading back to I.

If C, F and G keep showing up together, it's not coincidence. They're I, IV, V of the same key. They were always going to belong together.

05The pattern is keyless

The formula doesn't care which key you're in. Move to G major. The notes change. The pattern doesn't.

Same W-W-H-W-W-W-H. The half steps now sit between B–C and F♯–G. Stack the same way and you get:

This is why the formula matters. Learn one pattern, transpose to every key. Don't memorize twelve sets of seven chords — derive them.

06Quiz · Name the quality

Given a key and a scale degree, what is the chord's quality? Six rounds. Click an answer.

07Test bank · 30 shuffled

Drill against thirty randomized questions across all twelve keys. Hit reset to reshuffle.

08Optional · 50-question deep test

For when the basic pattern is locked in. Same diatonic system, three question types in random order: name the quality, name the chord, and spot the chord that doesn't belong in the key. All twelve major keys included.