The three chord functions

Three jobs, seven chords

In any major key, every diatonic chord has one of three functions:

• Tonic (T) — home, rest, stability. I, iii, vi are tonic-family.
• Subdominant (SD) — motion away from home. ii, IV are subdominant-family.
• Dominant (D) — strong pull back home. V, vii° are dominant-family.

The “T → SD → D → T” sequence is the gravity of music. Every song is a journey through these functions.

How chords cluster

The chords inside each family share enough notes to substitute for each other:

Family	Chords in C major	Shared anchor notes
Tonic	C, Em, Am	Contain E (the 3 of C)
Subdominant	Dm, F	Contain F (the 4 of C)
Dominant	G, Bdim	Contain B (the 7 of C)

The vi chord (Am) is the most powerful tonic substitute — it shares two notes with the I (Am = A C E; C = C E G). Replace a I with a vi and the song takes a melancholic turn but keeps its sense of resolution.

Visualising the journey

This is the basic cadence — the engine of most Western music. Notice it goes T → SD → D → T, never D → SD (going backwards through the cycle is rare and used for special effect).

Common cadence types

A cadence is the end of a phrase or section, where the chords resolve.

Cadence	Pattern	Feel
Authentic	V → I	Strong, conclusive (“the end”)
Plagal	IV → I	Gentle, “amen”, peaceful end
Half	… → V	Pause, suspense (phrase ends on dominant)
Deceptive	V → vi	“Almost home” — surprise, bittersweet

Listen for these endings in songs you know. The chorus of “Let It Be” ends with a plagal cadence. The end of “Twinkle Twinkle” is an authentic cadence. Most TV theme song mid-phrases end on a half cadence to set up the resolution at the end.

Why this works

The V chord contains the leading tone (the 7 of the scale) — and the leading tone, a half-step below the tonic, pulls hard upward. Specifically, in a V7 chord, the 3rd (which is the scale’s 7) pulls up to the 1, and the ♭7 (which is the scale’s 4) pulls down to the 3 of the I chord. That double resolution is the magic.

This is also why a V7 → I cadence works so much harder than v → i. The dominant 7 has the resolving tendencies baked in.

Try this

Play C → F → G → C. Authentic cadence at the end. Sounds finished.

Now play C → F → C. Plagal cadence. Sounds peaceful, but slightly suspended — like the song could go either way.

Now play C → F → G and stop. Half cadence. Suspense. The phrase clearly isn’t over; the listener is waiting for the next bar.

Now play C → F → G → Am. Deceptive cadence. The V “should” go to I (C), but resolves to vi (Am) instead — bittersweet.

These four cadences are a complete emotional toolkit. You’ll use them forever.