Harmonic function

Seven chords is a lot to reason about. In practice they sort into three categories by what they tend to do next.

The typical circuit is tonic → predominant → dominant → tonic, and an enormous amount of music is that loop with decoration.

1. CITonic
   →
2. FIVPredominant
   →
3. GVDominant
   →
4. CITonic

Why the groupings are what they are

Tonic chords share notes with I. In C major, I is C E G, vi is A C E and iii is E G B. Each shares two of its three notes with the tonic triad, so each can stand in for it — which is why a progression can land on vi and feel like a soft, incomplete arrival rather than a real departure.

Dominant chords contain the leading tone. V (G B D) and vii° (B D F) both contain B, a semitone below C. That semitone is the pull. vii° also contains F, making a tritone with B — the same tritone that lives inside V7.

Predominant chords contain the fourth degree. IV (F A C) and ii (D F A) both contain F, and neither contains the leading tone. They sit comfortably between home and the dominant.

Direction of travel

Functions have a strong conventional order:

Tonic → Predominant → Dominant → Tonic

Moving forwards along that chain sounds natural. Moving backwards — dominant to predominant, V to IV — sounds like a retreat. That is not a prohibition: the V–IV move is completely standard in blues and rock, where it is used precisely because it sidesteps the expected resolution.

What this buys you

You stop memorising progressions and start hearing categories. I–vi–IV–V and I–vi–ii–V are the same functional shape — tonic, tonic substitute, predominant, dominant — with one chord swapped for another doing the same job.

It also explains why substitution works. Any predominant can replace any other predominant and the progression still makes sense, because the function is what the ear is tracking.

In minor keys

The same three functions apply, with one wrinkle: the natural minor's v chord is minor and has no leading tone, so it does not pull. Composers raise the seventh degree to make V major, which is why the harmonic minor exists.

So in A minor you will see Am (tonic), Dm (predominant), E major (dominant, with G♯ borrowed from harmonic minor), Am.