The chords inside a scale

Scales supply notes; keys supply a home

A scale is an ordered set of notes. A key is a musical context in which a note and its chord feel central. C major and A natural minor share seven notes, but C and Am are different homes. Looking only at the white keys cannot tell you which chords create a particular sense of direction.

For our first experiment, choose C major and establish C as home by starting and ending on a C chord. This is a listening experiment, not a rule that every song must begin or end on I.

Where do major and minor come from?

In C major, C–E–G has distances 0–4–7: a major triad. D–F–A has 0–3–7: a minor triad. Both use alternate scale notes, but the semitone pattern gives them different thirds. Starting on B gives B–D–F, 0–3–6: a diminished triad.

These distances are measured from each chord’s own root, not from C in every example. A bare letter such as C denotes major here; m denotes minor, and dim or ° denotes diminished.

A Roman numeral is a chord’s address in the key

Start with C major: C, D, E, F, G, A, B. Number the notes 1–7. F is note 4. Build a chord from F using alternate scale notes: F–A–C. This is F major. IV means this chord on degree 4, not four notes or the fourth chord you play.

Uppercase numerals mean major here, lowercase minor, and ° diminished. Degree 2 gives D–F–A: Dm, or ii. The major-key pattern is I–ii–iii–IV–V–vi–vii°: C, Dm, Em, F, G, Am, Bdim in C major.

One chord, different roles

Play G–B♭–D. From G, the distances are 0, 3 and 7 semitones: a minor triad, Gm. This identifies the chord. It does not yet identify the key of the music.

Why is Gm ii in F major? First write the F major scale: F–G–A–B♭–C–D–E. G is the second note. Build a triad from G using alternate scale notes: G–B♭–D. Its root is on degree 2, giving II; because the chord is minor, use lowercase: ii. B♭ alone does not determine this role — we have chosen F as the tonal centre.

In G minor, G is the centre and the same Gm chord is i. In B♭ major, G is degree 6, making Gm vi. The chord’s notes and name stay the same; its numeral depends on the key. The inversion B♭–D–G is still Gm and still ii when we are in F major.

Try Gm–C7–F, then Gm–Cm–D7–Gm. In the first, Gm can begin ii–V7–I in F major. In the second, Gm is home in G minor; D7 includes F♯, the raised leading tone often used in minor to point back to G. Hold the final chord a little longer and compare where the music settles.

A tool must therefore distinguish “you are playing Gm” from “Gm could be ii in F major”. The latter is a possible context. We need more of the progression, or a key you choose, before applying that numeral to explain your playing.

Which chords should I start with?

Build a triad on each scale note using alternate notes of the scale. C major gives C, Dm, Em, F, G, Am and Bdim. These are diatonic chords: they use only the scale notes of the key. They provide a manageable first palette, not a list of the only permitted chords.

Start with C, F and G: I, IV and V. Play C–F–G–C, four beats each. C establishes home, F moves away and G can point back towards C. Then try C–Am–F–G. Am shares C and E with the C chord and adds a softer minor colour while staying in C major.

Chords within a key are not equally stable, and shared scale notes do not make every order equally convincing. Melody, rhythm, bass and expectations from what came before all contribute.

The same pattern in a new key

I–IV–V–I becomes C–F–G–C in C major. In G major, count from G: G, A, B, C, D, E, F♯. Now I = G, IV = C and V = D: G–C–D–G. The C chord is I in one key and IV in the other. A numeral describes a relationship to the key rather than a fixed piano key.

Try exercises 05 and 06 in Practice. Say both the chord name and numeral as you listen. Exercise 07 adds vi and ii: minor chords built from the same major scale.

A practical route from melody to chords

1. Find a likely home: which note makes the melody feel finished? Examine its notes and try a suitable major or minor key. The first or final note is a clue, not proof.

2. Choose a gentle harmonic rhythm, perhaps one change per bar. Pay particular attention to long melody notes and accented beats. Start with chords from the key that contain those notes. In C major, melody note E can sit over C = C–E–G, Am = A–C–E or Em = E–G–B. Each has a different colour.

3. Play the melody over each candidate and listen to the chords before and after it. For E–F–G–E, one long note per bar, try C–F–G–C. That is one possible accompaniment, not the correct answer.

4. Use inversions to make the changes easier. A short melody note between chord tones may be a passing note; you do not need a new chord for every note. Even long non-chord tones can provide tension when chosen by ear.

The root is not always the bass note

C–E–G and E–G–C are both C major. C remains the root even when E is the lowest note in the second version, written C/E.

For a single note, tonic means the key’s centre. The tonic chord is the chord on that centre. A G chord in C major therefore has G as its root while the key’s tonic remains C.

Add a note and hear the direction

Stack another third to make a seventh chord. C–E–G–B is Cmaj7; G–B–D–F is G7. The B and F in G7 often move to C and E in a C chord. This creates a clear opportunity for tension and resolution. It is a common movement, not an obligation.

A seventh spans seven letter names. In Cmaj7, B is 11 semitones above C: a major seventh. In G7, F is 10 semitones above G: a minor seventh. Thus maj7 and 7 mean different things. A major triad with a minor seventh is called a dominant seventh chord. In C major, G7 can function as the dominant and lead back to the tonic chord C.

When a chord borrows an outside note

D major contains F♯, so it is not a diatonic triad in C major. Yet C–D–G–C can work: D can point to G, which points to C. Here D is a secondary dominant, V of V. The outside note contributes to the direction rather than automatically being a mistake.

Begin with diatonic chords to hear their relationships, then expand one chord at a time. The circle of fifths helps explain connections between roots; it does not say that only neighbouring chords work together.

Try it at the piano

Try exercises 05–07 in Practice: C–F–G–C, the same degrees in G major, then minor chords. Sing E–F–G–E over C–F–G–C, one note per bar. Listen for where melody and harmony feel at home.