10. Diatonic Chords: Building Progressions from a Scale

Stack thirds on each note of a major scale and you get seven chords. Their qualities always land in the same order, in every key, without exception: I major, ii minor, iii minor, IV major, V major, vi minor, vii diminished.

That pattern is the reason chords in a song sound like they belong together, and the reason a progression can be moved to any key and still work.

The seven chords in three keys

Play any combination of a row and it will sound coherent. That is all "in key" means.

Why the qualities fall that way

Nothing arbitrary is going on. A major scale has semitones between the 3rd and 4th degrees and between the 7th and 1st. Build a triad on each degree and those two narrow gaps land in different places each time, producing major, minor, or diminished depending on where they fall. Since every major scale has the same gap pattern, every major key gets the same chord qualities.

Minor keys

Natural minor uses the same seven notes starting from the sixth degree, so the pattern rotates: i minor, ii diminished, III major, iv minor, v minor, VI major, VII major. In A minor that is Am, Bdim, C, Dm, Em, F, G.

In practice the v is usually played major. Am–Dm–Em–Am is correct but limp; Am–Dm–E–Am has a real pull home, because raising the third of that chord creates a leading note a semitone below the tonic. Most minor-key music is not strictly diatonic for exactly this reason.

Using it to learn songs by ear

Knowing the key narrows the next chord to seven candidates before you hear it. In practice it narrows further, because some are far more common than others: I, IV, V, and vi carry most songs, while iii and vii° are comparatively rare.

When a chord genuinely will not fit, it is usually one of three things:

• A borrowed major V in a minor key — by far the most common exception.
• A secondary dominant — a major chord aiming at a chord that is not the tonic, such as an E7 leading into Am in the key of C.
• A slash chord — an ordinary diatonic chord that looks strange because of the note underneath it.

Try it

Pick a key and write a four-chord progression using only its seven chords, starting and ending on I. Look each chord up in the Chord Dictionary and choose voicings that sit near each other on the neck rather than the first shape you know. Diatonic chords share notes with their neighbours, so with the right voicings your fingers will barely move between changes — and that smoothness is a large part of why these progressions sound settled in the first place.

The chords you will actually meet

All seven are available, but songs do not use them evenly. In a major key, I, IV, V and vi carry the overwhelming majority of popular music. The ii turns up constantly in jazz and in any progression heading towards V. The iii is uncommon and tends to appear as a passing chord between I and IV. The vii° is rare in guitar music altogether — its job is usually done by the V7, which contains the same tritone and is far easier to play.

That ordering is useful when you are working something out by ear. If you know the key and the chord is not I, IV, V or vi, check ii next and only then start considering the unusual ones.

Seventh chords from the same scale

Stack one more third on each degree and you get the diatonic seventh chords, which follow their own fixed pattern: Imaj7, iim7, iiim7, IVmaj7, V7, vim7, viim7b5. In C that is Cmaj7, Dm7, Em7, Fmaj7, G7, Am7, Bm7b5.

Notice that only one chord in the whole key is a plain dominant 7th — the V. That is why a G7 announces the key of C so strongly: no other chord built from those seven notes has that shape. It is also why the ii–V–I cadence works so reliably, since Dm7–G7–Cmaj7 walks straight through the strongest pull the key contains.