Keys, scales, and chord families
A key is a family of 7 notes plus the 7 chords built from them. Once you see how the family is assembled, every progression in pop, rock, folk, and jazz starts to look familiar.
What a 'key' actually means
A key picks one note as 'home' (the tonic) and chooses six other notes that consistently sound good with it. Together those seven notes are the key's scale. From those seven scale notes you can build seven chords — one rooted on each scale degree — and those seven chords are the key's chord family.
When you say a song is 'in G major', you're saying: it draws almost all its melody from the G major scale, and almost all its chords from the G major chord family. Songs rarely use all seven; most pop songs use four of them.
The major-key formula
A major scale is built by the step pattern W-W-H-W-W-W-H (whole step, whole, half, whole, whole, whole, half). Apply it to any starting note and you get that note's major scale. C major hits no sharps or flats (the only major key that does); every other key needs at least one accidental to keep the pattern intact.
W = whole step (2 frets) · H = half step (1 fret)
The major-scale step pattern. Memorize this — it's the ruler everything else is measured against.
Building the chord family
Stack two more notes on top of each scale degree, each one a third above the last. The result is the seven diatonic triads. In every major key the quality pattern is identical: I major, ii minor, iii minor, IV major, V major, vi minor, vii° diminished.
Notice: only three chords (I, IV, V) are major. Three are minor (ii, iii, vi). One (vii°) is the spiky diminished chord that almost no song actually rests on.
Diatonic chords in C major
C major's diatonic family. Same pattern in every major key.
Same pattern, different key
Move the same pattern to G major and you get a different set of names but the same shape: G, Am, Bm, C, D, Em, F♯°. To D major: D, Em, F♯m, G, A, Bm, C♯°. The chords change, the roles don't — I is still home, V is still the chord that wants to resolve to I.
Diatonic chords in G major
G major. Same Roman roles, shifted up a perfect 5th.
Minor keys
Natural minor uses the pattern W-H-W-W-H-W-W and yields a different family: i minor, ii° diminished, III major, iv minor, v minor, VI major, VII major. In A minor: Am, B°, C, Dm, Em, F, G.
Most minor-key songs sharpen the 7th degree when they reach the V chord, turning v (minor) into V (major). That sharpened leading tone is what gives minor-key cadences their dramatic pull back to the tonic. The result is called the 'harmonic minor' scale.
Diatonic chords in A minor
A minor's natural-minor family. The relative minor of C major — same seven notes, different home.
Why every major has a 'relative minor'
C major and A minor use the exact same seven notes. The only difference is which note feels like home. That pairing is true for every key: each major has a relative minor a m3 below it, and they share the same key signature. G major / E minor. D major / B minor. A major / F♯ minor.
This is why you can take any major-key chord progression, treat the vi chord as the new home, and instantly have a minor-key song.
Once you know the key, you can guess the chords. Reverse it: given a chord progression, the chord that feels like 'home' (often the first or last chord) is almost always the I.
How to find the key of a song
- Look at the most-used chords. If three of them are major and two are minor in the I–IV–V–ii–vi pattern, you've almost certainly found a major key.
- Check the bass note of the chord the song ends on — usually the tonic.
- Look at any accidentals (sharps or flats in the chord names). If they all match a single major key's signature, that's probably the key.
- If the song feels 'sad' and the I chord is minor, you're in the minor key — and the relative major shares the same chords (so the same family applies, just centred differently).
Practice this week
- Pick three songs you can play. For each one, write down all the chords and try to identify the key.
- Play the C major family (C, Dm, Em, F, G, Am) back-to-back. Then play just I–V–vi–IV (C–G–Am–F). That's the most-used progression in popular music — and you now know exactly why each chord is there.
- Transpose 'I–V–vi–IV' to G (G–D–Em–C) and to D (D–A–Bm–G). Same roles, different shapes.