Harmonic function, and the three places a chord can be
Every chord in the harmonic field plays one of three roles — rest, departure or tension — and it is the role, not the name, that decides what it can do in a song.
The harmonic field hands over seven chords. A song does not use them as seven different things: it uses them as three. Each degree plays a role, and there are only three roles, which are called functions. Once you hear the function, most of tonal harmony becomes readable.
The three roles
Tonic is rest. The first-degree chord is the key's home; the music leaves it and returns to it, and a phrase that ends there really ends.
Dominant is tension. The fifth-degree chord contains the note that most wants to rise to the tonic, the leading tone, and, with the seventh, the tritone that asks to resolve. It cannot sit still.
Subdominant is departure. The fourth-degree chord leaves home without creating tension; it is the road between rest and the dominant.
Tonic, subdominant, dominant, tonic. That four-chord round trip is the most basic sentence in tonal music, and each chord in it is doing a different job.
Why the dominant pulls
What gives the fifth degree its tension is not its pitch: it is two notes inside it. In C major, the G chord with a seventh has B and F.
B is the leading tone, a semitone below C, and wants to rise. F is a semitone above E, and wants to fall. Together they form the tritone, , and they resolve in opposite directions, to C and E, which are the root and the third of the tonic.
The dominant is dominant because of that pair. A chord containing the key's leading tone and its fourth degree asks for the tonic, whatever its name.
The other degrees have relatives
Seven chords, three functions: some degrees share a role. The practical rule is that chords with two notes in common tend to work alike.
The sixth degree, A minor, has A, C and E; the tonic has C, E and G. Two notes in common, and the sixth degree sounds like a tonic in a different colour. The third degree, E minor, also shares two notes with the tonic.
The second degree, D minor, has D, F and A; the subdominant has F, A and C. Two in common: the second degree is subdominant. The seventh degree, B diminished, has B, D and F; the dominant with a seventh has G, B, D and F. Three in common, tritone included: the seventh degree is dominant.
The picture comes out like this:
- Tonic: I, with vi and iii as relatives.
- Subdominant: IV, with ii as a relative.
- Dominant: V, with vii° as a relative.
Substituting without changing the sentence
If two chords fill the same function, one can take the other's place without the phrase losing its sense. Swap the subdominant for the second degree, and the closing tonic for the sixth:
The sequence of functions is still rest, departure, tension, rest. The chords changed, the sentence did not. Much of reharmonisation comes from here: the function is what the music is asking for, and there is more than one chord that answers each request.
Hearing the function
Sing the root of each chord in a simple song and ask, at each one, whether it is at home, away from home, or wanting to come back. Those three answers are the three functions, and the ear recognises them before the name exists. The name only organises what is already heard.
What to take from here
Seven chords, three roles. The tonic rests, the subdominant departs, the dominant tenses because of the tritone it carries, and the remaining degrees group around those three by what they have in common with them. Reading a song by function is seeing the sentence instead of the chords; cadences and the II–V–I are the shortest sentences there are.