Where ii, vi and iii Sit: The Chords That Stand In

A major key has three major chords and three minor ones, plus the diminished chord on the seventh degree. The first lesson in this module used the three major chords to explain the three functions, because they are the plainest example of each.

The three minor chords are not a second, sadder set. They are substitutes: each one does a job another chord already does, with different colour.

Two chords can stand in for each other when they share most of their notes. That is not a metaphor — it is arithmetic, and in a major key it works out remarkably cleanly.

In C major:

Chord	Notes	Shares with
I	C E G	—
iii	E G B	two with I
vi	A C E	two with I
IV	F A C	—
ii	D F A	two with IV

vi and iii each share two of three notes with the tonic. That is why they can act as tonic: land on one instead of I and the music has arrived somewhere very close to home, without quite closing the door.

ii shares two of three with IV. That is why it acts as subdominant, and why ii–V–I and IV–V–I are the same sentence.

The dominant has no substitute among the minor chords, and that is not an oversight: the leading tone is what makes a dominant a dominant, and only V and vii° contain it.

vi: the tonic that does not settle

vi is the relative minor — the same seven notes as the major key, heard from a different home.

Used as a tonic substitute, it is what makes the deceptive cadence work: V wants I, and vi arrives instead. Two of the three notes the ear expected are there, so it is not a jolt; the missing one is the tonic itself, so it is not a resolution either.

It is also why I–V–vi–IV loops forever. The vi is where the ending should have been.

ii: the subdominant with momentum

ii does IV’s job and moves better, for the reason the last lesson gave: from ii, the roots fall by a fifth twice in a row into the tonic.

There is a second, quieter reason to reach for it. ii is minor and IV is major, so swapping one for the other changes the colour of the departure without changing what the phrase is doing. A songwriter picking between them is choosing a shade, not a function.

iii: the awkward one

iii is the least used of the seven, and it is honest to say why rather than pretend it is a hidden gem.

It shares two notes with I, so it can substitute for the tonic — but it also contains the leading tone, which gives it a hint of dominant that a tonic substitute does not want. It sits between two functions without committing to either.

Where it does work is as a passage: I–iii–vi walks the bass down through the key in thirds, and iii is the step in the middle. Used as a stopping point it is vague; used as a step it is graceful.

What this buys you

Take I–IV–V–I and substitute: I–ii–V–I, or vi–IV–V–I, or I–vi–ii–V. Every one of them is the same sentence — home, away, tension, home — recoloured.

That is the technique. The functions are the structure and the substitutes are the palette, and knowing which chord stands in for what is the difference between changing a progression’s colour and breaking its shape.

Common mistakes to avoid

Treating the minor chords as “the sad ones”. In a major key they are structural, not emotional. vi is where a deceptive cadence lands, not where a song gets sad.
Expecting a substitute to be interchangeable everywhere. vi substitutes for I at an ENDING. Starting a phrase on vi makes the key sound minor instead.
Reaching for iii to sound sophisticated. It is rare because it is ambiguous, and it works best passing rather than landing.

Now play

The drill plays a progression and asks which chord the last one was. Every option is a degree of the key, and the minor ones are in the mix now.

The question to ask yourself is the same as in the first lesson — did it settle, leave, or pull? — with one addition: if it settled, did it settle all the way? That is what separates I from vi.