Tonal functions – free music theory course

The seven chords of a key don't all play the same role. They fall into three broad functions, like the characters in a story:

Function	Role	Main chord	Substitutes
Tonic (T)	rest, stability, “home”	I	vi, (iii)
Subdominant / predominant (S)	moving away, preparation	IV	ii
Dominant (D)	maximum tension, pull to the tonic	V, V7	vii°

The typical tonal phrase tells the story: departure (T) → moving away (S) → tension (D) → return (T).

M:4/4
L:1/4
%%staves {RH LH}
V:RH clef=treble
V:LH clef=bass
K:C
[V:RH] "C"[Ec]2 "F"[Fc]2 | "G7"[FB]2 "C"[Ec]2 |]
[V:LH] "_T"[C,G,]2 "_S"[F,A,]2 | "_D"[G,D]2 "_T"[C,C]2 |]

Why does the dominant pull so hard?

The V chord contains the leading tone (B in C), a semitone below the tonic: it “wants” to rise. Adding the seventh (G7) brings in F, which wants to fall to E. Together these two notes form a tritone, the most unstable interval: its resolution onto the C chord produces that unmistakable sense of arrival.

Substitutes: cousin chords

Two chords sharing two notes can often replace each other. Am (A–C–E) shares C and E with C major: it has a tonic function, softer and more wistful. Dm (D–F–A) shares F and A with F major: it's a predominant, very often used before V (the famous ii–V–I). B° (B–D–F) sits inside G7: it's a dominant without its root.

Listen for function rather than chord: sing the tonic while someone plays a progression. Over tonic chords it sounds “right and restful”; over the dominant it rubs and makes you want to move.

Going backwards is rare: in classical music, S → D is extremely common, D → S much rarer. A V followed by IV gives a “blues” or “rock” effect (which is precisely why it's so common in those styles!).