Chord Function: Tonic, Subdominant, Dominant

The idea

Every diatonic chord plays one of three roles.

Function	Chords	Feeling
Tonic (T): home	I, vi, (iii)	Stability, arrival, rest
Subdominant (S): departure	IV, ii	Motion away from home: “we are going somewhere”
Dominant (D): tension	V, vii°	Maximum pull back to the tonic

The dominant chords pull because they contain the leading tone, degree 7, which wants to rise to 1. V7 also contains degree 4, which wants to fall to 3. Together they form the tritone from lesson 3.2.

The universal grammar is T → S → D → T. Home, away, tension, home. Almost every progression you love is a path through this loop, sometimes skipping a step, sometimes going around twice.

Substitution. Chords in the same family share two of their three notes, so they can replace each other: vi for I, ii for IV, vii° for V. The function stays and the color changes. iii is the ambiguous one: it shares notes with both I and V, and usually acts as a weak tonic.

Cadences are the punctuation of music: how phrases end.

Cadence	Chords	Effect
Authentic	V → I	Full stop. Strongest with V7.
Plagal	IV → I	“Amen”. A softer landing, huge in rock and gospel.
Half	anything → V	Comma. The phrase pauses on tension. A verse that ends here begs for the chorus.
Deceptive	V → vi	Promises home and delivers its sad cousin. Extends the phrase with instant poignancy.

Listen to all four. Each player ends on the cadence:

On your instrument

Feel the functions physically. Play these on guitar in G and stop on each chord for four slow beats. Ask one question each time: could the song end here?

• G (I): yes. Complete rest.
• C (IV): no, but there is no urgency. You are away from home.
• D7 (V7): no. It has to move.
• Em (vi): almost. It rests, but it is sad about it.

Piano. Play C major in the right hand. Move only the top two notes to make F/C (C F A), then G/B below (B D G), then back to C. You can feel in your hand how little has to move for the function to change.

Worked example

Functions in three familiar progressions:

Progression	Functions	What it does
I – IV – V – I	T – S – D – T	The full loop, once
I – vi – IV – V	T – T – S – D	Stays home for two chords, then leaves, and ends on tension so the loop restarts
vi – IV – I – V	T – S – T – D	Starts on the minor tonic and visits the major one in the middle

The second and third lines never land on V → I inside the loop. They end on V, a half cadence, and the resolution arrives when the loop starts again. That is why four-chord loops can repeat for a whole song without feeling finished.

Exercises

1. Label T, S or D on every chord in the five songs you converted to numerals in lesson 4.1. Find one deceptive cadence somewhere in your listening this week.
2. Write two 8-bar progressions in G major: (a) ending with a half cadence, (b) with the same first six bars, ending with an authentic cadence. Play them back to back and feel the question and the answer.
3. Take a progression you wrote. Replace one I with vi and one IV with ii. Compare the two versions.
4. Play V7 – I and then V7 – vi in four keys. Sing the top note of each chord so your ear learns the difference between arriving and being deceived.

Common mistakes

• Ending every phrase on I. If every line of the verse arrives home, there is no reason to keep listening. Save the authentic cadence for the moments that need it.
• Treating substitution as a rule. vi can replace I, but the feeling changes. Use it because you want that change.

Check yourself