In Why I-IV-V you met the three pillar chords. But real songs are full of other chords — vi, ii, iii — that somehow fit right in. Here's the trick that lets them: if two chords do the same job, you can swap one for the other. That swap is called a substitution.
Chords have jobs, not just names
Back in Roman numerals we numbered the seven chords of a key. Those seven sort into just three jobs — the same tonic / subdominant / dominant roles that drive every progression:
| Job | Feeling | Main chord | Substitute(s) |
|---|---|---|---|
| Tonic | home, at rest | I (C) | vi (Am), iii (Em) |
| Subdominant | leaving home | IV (F) | ii (Dm) |
| Dominant | tension, pulling home | V (G) | vii° (Bdim) |
A progression is really a sequence of jobs — home → away → tension → home. As long as each slot is filled by a chord with the right job, the story still makes sense. Which chord you pick just changes the colour.
Why the swap works: shared notes
You can't substitute just any chord. The reason a substitute sounds like the chord it replaces is that they share most of their notes. Each of these pairs is a triad of three notes that overlaps its partner by two:
| Job | Pair | Notes | Shared |
|---|---|---|---|
| Tonic | I = C / vi = Am | C E G / A C E | C, E |
| Subdominant | IV = F / ii = Dm | F A C / D F A | F, A |
| Dominant | V = G / vii° = Bdim | G B D / B D F | B, D |
Look at the subdominant pair up close — F major and D minor share two of their three notes:
Two out of three notes in common means most of the sound is identical — so the ear accepts Dm in a spot that wanted F. That two-note overlap is the whole reason substitution works.
The rule in one breath. Same job + shared notes = safe swap. If two chords have the same function and share two notes, you can trade one for the other and the progression still flows.
Hear it: IV → ii
Take the plainest progression there is and substitute the subdominant. IV becomes its partner ii — same job, two shared notes — and the line gains a touch of colour without changing course:
Notice the second chip stays green in both — the job didn't change, only the chord filling it. That's the point of the colour coding: substitution swaps the note, keeps the function.
The two subs worth memorising
Of all the pairs, two are used constantly, and they're the same pairs you already know as relative keys — a major chord and the minor chord a minor 3rd below it:
- I ↔ vi (C ↔ Am) — swap bright home for its darker cousin. Landing on vi where you expected I is the famous deceptive move (more on that in Sevenths & cadences).
- IV ↔ ii (F ↔ Dm) — the sub you just heard. Using ii → V → I instead of IV → V → I makes the roots fall by fifths (D → G → C), the strongest root motion there is.
Add the tonic substitute vi to the three pillars and you get the four chords that run pop music — which is exactly where we go next.
In one line: chords that share a function and share notes can stand in for each other, so swapping IV for ii or I for vi recolours a progression without breaking its home → away → tension → home flow.