Lessons · Intermediate
Diatonic Harmony - Chords in a Key and Roman Numerals
Which chords belong to a key, how to name them with Roman numerals, and what job each chord does.
Why do some chords sound "right" together in a song while others sound out of place? Most of the time it is because the chords come from the same scale. Diatonic harmony means building chords using only the notes of one key. Once you know the pattern, you can predict the chords of any song in any key, transpose instantly, and understand why progressions work.
Before you start, make sure you know the major scale, triads and seventh chords.
Building triads on every scale degree
Take the C major scale - C D E F G A B - and stack a triad on each note, using only notes of the scale (every other note):
| Degree | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|---|
| Notes | C E G | D F A | E G B | F A C | G B D | A C E | B D F |
| Chord | C | Dm | Em | F | G | Am | Bdim |
| Quality | major | minor | minor | major | major | minor | diminished |
| Numeral | I | ii | iii | IV | V | vi | vii° |
This pattern - major, minor, minor, major, major, minor, diminished - is identical in every major key, because every major scale has the same step pattern.
Roman numerals
Roman numerals name a chord by its scale degree instead of its letter name:
- Uppercase (I, IV, V) = major triad
- Lowercase (ii, iii, vi) = minor triad
- Lowercase with ° (vii°) = diminished triad
- Uppercase with + (III+) = augmented triad
The great advantage: numerals work in every key. "I - vi - IV - V" is C - Am - F - G in C major, G - Em - C - D in G major and E - C#m - A - B in E major. Musicians use numerals to talk about progressions, transpose on the fly, and analyse songs.
| Key | I | ii | iii | IV | V | vi | vii° |
|---|---|---|---|---|---|---|---|
| C | C | Dm | Em | F | G | Am | Bdim |
| G | G | Am | Bm | C | D | Em | F#dim |
| D | D | Em | F#m | G | A | Bm | C#dim |
| A | A | Bm | C#m | D | E | F#m | G#dim |
| E | E | F#m | G#m | A | B | C#m | D#dim |
| F | F | Gm | Am | Bb | C | Dm | Edim |
| Bb | Bb | Cm | Dm | Eb | F | Gm | Adim |
Diatonic seventh chords
Add the next scale note on top of each triad and you get the diatonic seventh chords:
| Degree | I | ii | iii | IV | V | vi | vii |
|---|---|---|---|---|---|---|---|
| C major | Cmaj7 | Dm7 | Em7 | Fmaj7 | G7 | Am7 | Bm7b5 |
| Numeral | Imaj7 | ii7 | iii7 | IVmaj7 | V7 | vi7 | viiø7 |
| Quality | maj7 | m7 | m7 | maj7 | 7 | m7 | m7b5 |
Note again: only one dominant seventh chord (V7) exists in a major key. When you hear G7, the music is very likely heading to C.
Harmonic functions: tonic, subdominant, dominant
Each chord has a job. Theory groups the diatonic chords into three functions:
| Function | Feeling | Main chord | Substitutes |
|---|---|---|---|
| Tonic (T) | home, rest, stability | I | vi, iii |
| Subdominant (S) / predominant | moving away, preparing | IV | ii |
| Dominant (D) | tension, wants to resolve | V (V7) | vii° |
Chords that share a function share notes. Am (vi) has two notes in common with C (I), so it can replace the tonic. Dm (ii) shares two notes with F (IV). Bdim (vii°) is G7 without its root, so it acts as a dominant.
Why the dominant pulls so hard
The V chord contains the leading tone (B in C major), a half step below the tonic. V7 adds the 4th degree (F), which forms a tritone with B. On resolution, B rises to C and F falls to E - both by half step. This double pull is the engine of tonal music.
The basic cycle: T - S - D - T
Most progressions follow the cycle tonic - subdominant - dominant - tonic:
- I - IV - V - I (C - F - G - C)
- I - ii - V - I (C - Dm - G - C)
- I - vi - ii - V (C - Am - Dm - G)
Going from dominant back to subdominant (V - IV) is common in blues and rock, where it sounds natural, but it is a weaker move in classical harmony. For endings and phrase types see cadences.
Chords in a minor key
Natural minor
Build triads on A natural minor (A B C D E F G) and you get the same chords as C major, but starting from Am:
| Degree | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|---|
| Chord | Am | Bdim | C | Dm | Em | F | G |
| Numeral | i | ii° | III | iv | v | VI | VII |
| Seventh chord | Am7 | Bm7b5 | Cmaj7 | Dm7 | Em7 | Fmaj7 | G7 |
The pattern in every minor key: minor, diminished, major, minor, minor, major, major.
The harmonic-minor V
In natural minor the v chord (Em) is minor and has no leading tone, so it resolves only weakly. In practice, minor-key music almost always raises the 7th (G to G#), borrowing from harmonic minor. This changes three chords:
| Degree | Natural minor | With raised 7th (harmonic minor) |
|---|---|---|
| III | C | C+ (C E G#) - rare |
| v | Em | E (E G# B), E7 (E G# B D) |
| vii | G | G#dim (G# B D), G#dim7 (G# B D F) |
The major V and V7 give minor keys a strong dominant, just like in major. The resulting standard minor-key set is: i - ii° - III - iv - V - VI - vii°, with VII (G) still used too, especially in rock and folk.
Minor-key seventh chords and functions
- Tonic: i (Am, Am7), VI (F)
- Subdominant: iv (Dm7), ii° (Bm7b5)
- Dominant: V7 (E7), vii°7 (G#dim7)
The minor ii - V - i - Bm7b5 - E7 - Am - is the minor-key counterpart of the major ii - V - I.
ToolsChord Progression PlayerPlay common chord progressions in any key and tempo to practise, jam or write.Finding the key of a song
Diatonic harmony also works in reverse. If a song uses G, C, D and Em, look for the key that contains all four: G major (I, IV, V, vi). A major E chord appearing among Am, Dm and C points to A minor with a harmonic-minor V. The key finder does this for you, but doing it by hand is excellent practice.
Practice
- Write the diatonic triads and seventh chords in G, F and D major, with Roman numerals. Check: in F major, IV is Bb and vii° is Edim; in D major, V7 is A7.
- Write the chords of E minor and D minor, including the harmonic-minor V7. Check: E minor V7 = B7 (B D# F# A); D minor V7 = A7 (A C# E G).
- Play I - vi - IV - V and i - iv - V7 - i in three keys each.
- Label each chord in this progression with numeral and function: G - Em - Am - D7 - G. (Answer: I T, vi T, ii S, V7 D, I T in G major.)
- Take a simple song you know and write its chords as Roman numerals.
Keep exploring:
- Hear and transpose progressions with the progression player and the transposer.
- Find the key of any chord list with the key finder; see how keys relate on the circle of fifths.
- Drill chord spelling: triads 2 and sevenths 1.
- Bass players: walking ii-V-I and root-fifth patterns.
- Print the chord formulas chart and the circle of fifths poster.
- Key pages: C major, G major, A minor, E minor.
- Next lessons: cadences, chord progressions, chord inversions, modes.