Chord function

Chords in a key do not all do the same job. Some feel like home, some lead away, and some create tension. Knowing which is which tells you where a progression is going.

Three groups

Chords in a key fall into three functional groups. The tonic group is home (the tonic function); the subdominant group moves away and prepares, which is why it is also called predominant; the dominant group has tension and wants to resolve.

Chords in the same group share notes and can often replace each other: vi shares two notes with I, and ii shares two notes with IV. The iii chord is the least clear-cut: it shares notes with both I and V, and different books classify it differently. It is grouped with tonic here, as in the Key Chords tool.

Tension and release

The typical motion is T → S → D → T: leave home, prepare, build tension, come back in a release. Many common progressions are this cycle (lesson 8.5). The dominant pulls to the tonic because it contains the leading tone (7, a half step below the tonic) and, as a V7, the tritone (Module 10).

Root movements

The root movement between chords has a strength:

• down a 5th (= up a 4th): V→I, ii→V, vi→ii. The strongest;
• by step: IV→V;
• by 3rd: I→vi, the weakest change, because the two chords share notes.

In minor keys the groups are the same: i (and III, VI) tonic, iv and ii° subdominant, V and vii° dominant. VI can also act as a predominant before V.