Diatonic Chords & Roman Numerals
Build a triad on every note of a scale and you get the seven chords that belong to a key. Number them with Roman numerals and the same song reads the same in every key.
Builds on: 2.1 The Major Scale4.1 Triads: Three Notes, Four Moods
Seven notes, seven chords
A key is more than a scale — it is a family of chords. Take C major (C D E F G A B) and build a triad on each note using only notes from the scale: C–E–G, D–F–A, E–G–B, F–A–C, G–B–D, A–C–E, B–D–F. These are the diatonic chords of C major, “diatonic” meaning “from the key”. Nothing outside the scale is needed, so they all sound like they belong together — which is exactly why they do.
Look at their flavours. The scale’s uneven pattern of whole and half steps means the thirds come out in different sizes, and the chord qualities follow a fixed pattern that is the same in every major key:
Roman numerals: chords by their job
Naming chords by letter (C, Dm, Em, F, G, Am, Bdim) ties them to one key. Naming them by scale degree frees them: the chord on the first degree is I, on the second ii, and so on. The case carries the quality — upper case for major, lower case for minor, a small ° for diminished. Now “I–V–vi–IV” describes the same progression in C (C–G–Am–F), in G (G–D–Em–C) and in E♭ (E♭–B♭–Cm–A♭). Musicians who transpose on the spot think in numerals, not letters.
Three functions
The seven chords do not carry equal weight. They fall into three functions — three jobs a chord can do in a phrase:
- Tonic (I, and its stand-ins vi and iii) — home. Rest, arrival, the chord a piece begins and ends on.
- Subdominant (IV and ii) — away from home. Motion, preparation, a gentle lean forward.
- Dominant (V and vii°) — pulling home. Tension that demands resolution back to I. Both dominant chords contain the leading tone, the seventh degree that sits one semitone under the tonic and aches to rise into it.
Most progressions are a walk through these three: home → away → tension → home. I–IV–V–I is that walk in its purest form.
Build the same stack on the natural minor scale and the pattern comes out i ii° III iv v VI VII — and the v is minor, which pulls weakly. So composers raise the seventh degree (the harmonic minor from Unit 2) precisely to turn v into a major V and VII into vii°, giving minor keys a proper dominant. That is why A minor songs use E major, not E minor, before returning home. The lab has a switch for it.
C major and A minor share every chord: C major’s vi is A minor’s i. Reading a lead sheet, you can often tell which of the two you are in only by which chord the phrases keep coming home to.
In the lab, pick a key and press the seven numerals. Every chord is coloured by its function; listen for how I sits still, IV leans and V leans harder.
🎼 Lab — Harmonise the Scale
The seven triads of any key, one button each. Press them and hear the three functions.
- In C major play I, then IV, then V, then I: the walk home that underlies most music.
- Switch to A minor and toggle the minor flavour — hear how weak the natural v is compared with the harmonic V.
- Change key and press the same numerals: the letters change, the story does not.