Diatonic chords and Roman numerals – free music theory course

Build a triad on each of the seven degrees of a major scale, using only notes of that scale: you get the key's diatonic chords. To talk about every key at once, we label them with Roman numerals matching the scale degrees.

In major

Degree	Name	In C major	Quality	Seventh
I	tonic	C	major	Cmaj7
ii	supertonic	Dm	minor	Dm7
iii	mediant	Em	minor	Em7
IV	subdominant	F	major	Fmaj7
V	dominant	G	major	G7
vi	submediant	Am	minor	Am7
vii°	leading tone	B°	diminished	Bm7♭5

The pattern major – minor – minor – major – major – minor – diminished holds in every major key. In G major: G, Am, Bm, C, D, Em, F♯°.

M:4/4
L:1/4
K:C
"C""_I"[CEG] "Dm""_ii"[DFA] "Em""_iii"[EGB] "F""_IV"[FAc] | "G""_V"[GBd] "Am""_vi"[Ace] "Bdim""_vii°"[Bdf] "C""_I"[ceg] |]

Two notational conventions

The modern English-language convention uses upper case for major chords (I, IV, V) and lower case for minor ones (ii, iii, vi), with ° for diminished (vii°) and + for augmented. French and some older European textbooks write every degree in capitals (I, II, III…), leaving the quality implied by the key. We'll use the first convention from now on, since it tells you everything at a glance.

In minor

Minor is subtler, because the minor scale comes in several forms. With the natural minor (A minor: white keys from A to A), chord v is minor (Em): there is no real leading tone, a note a semitone below the tonic. That's why the 7th degree is raised (G♯ in A minor): the harmonic minor. We then get:

M:4/4
L:1/4
K:C
"Am""_i"[A,CE] "Bdim""_ii°"[B,DF] "C+""_III+"[CE^G] "Dm""_iv"[DFA] | "E""_V"[E^GB] "F""_VI"[FAc] "G#dim""_vii°"[^GBd] "Am""_i"[Ace] |]

In practice, composers take from both forms whatever suits them: i, ii°, III, iv, V, VI, vii° are the most common chords, with V (major) and vii° containing the leading tone, and III and VII (G major in A minor) borrowed from the natural form, especially in sequences and popular music.

Tip: in minor, if you see an accidental raising the 7th degree (G♯ in A minor, B♮ in C minor…), you are very probably on a dominant chord.

Careful: a Roman numeral names a degree, not a fixed chord. “V” is G in C major but D in G major. That's the whole point: a progression like I–IV–V transposes instantly.