Tonic, Predominant, and Dominant Function

Harmonic function describes the job a chord does, not just its letter name. A chord's function comes from which scale degrees it contains, especially whether it shares pitches with the tonic triad, the subdominant, or the leading tone. Chords built on 1, 3, and 6 share the tonic pitch itself, so they sound stable and settled: I, iii, and vi are tonic function. Chords on 2 and 4 share scale degree 4, the pitch that leans away from tonic, so ii and IV are predominant function, chords of departure and buildup. Chords on 5 and 7 share the leading tone, scale degree 7, which pulls hardest back to 1, so V and vii-degree are dominant function, full of tension demanding resolution.

These families move in one direction: tonic goes to predominant, predominant goes to dominant, dominant goes to tonic. That is why ii or IV sliding into V feels completely normal, predominant resolving into dominant tension, but V dropping back to IV feels backward, dominant retreating into predominant instead of resolving. In I vi IV V I, the vi substitutes for tonic, IV supplies predominant push, V builds dominant tension, and the final I delivers the arrival.

This cycle is exactly what makes a cadence legible. An authentic cadence ends V to I, dominant finally resolving into tonic, so the phrase feels closed. A half cadence stops on V, leaving the dominant's pull unresolved, so the phrase feels like it is holding its breath, waiting for the next phrase to finish the journey back to tonic.