Changing Key

Everything so far has assumed a song stays in one key and occasionally visits. Modulation is when it genuinely leaves and sets up home somewhere else.

How far it can go comfortably depends on how closely related the two keys are, and the circle of fifths is the map of that. Keys next door to each other on the circle share six of their seven notes, so moving between them is nearly seamless. Keys on opposite sides share very few, and jumping between those is a dramatic event.

The closely related keys of C major are G major and F major, its neighbours on the circle, plus their relative minors and its own, A minor. Those are the destinations most songs use.

Circle of Fifths

Find your key, then look at its immediate neighbours. Those are the keys a song can slip into without the listener noticing the join.

The smooth way to get there is a pivot chord. Find a chord that exists in both keys, treat it as belonging to the first key when you arrive at it, and as belonging to the second when you leave. The listener re-interprets it after the fact.

C major and G major share four chords — C, Em, G and Am. Any of them can be the hinge. Play a phrase in C, arrive on A minor, then follow it with D7, and the D7 announces G major, because D7 is the dominant of G and does not exist in C. The A minor was the pivot: the vi of C on the way in, the ii of G on the way out. Nothing jarred, and you changed key.

That is the general recipe, and it explains why the secondary dominants from lesson five matter so much here. A secondary dominant is a modulation that changes its mind immediately — it borrows the dominant of another key but resolves and stays home. Give the same chord a bar or two longer and let the new key settle, and the visit becomes a move.

The blunt alternative is the direct modulation: no pivot, no preparation, just shift everything up, usually by a semitone or a tone, most often for a final chorus. It is sometimes called the truck driver's gear change, which is not a compliment. It works because it is so abrupt that the listener hears it as an increase in energy rather than a change of key, and it is very hard to do without sounding cheap. Knowing it is available is worth more than using it.

One last relationship worth having. A change to the parallel minor — C major to C minor — keeps the root and changes the mode. It is not really a modulation to a distant key at all; it is the same borrowing from lesson six, taken to its conclusion, and it is why a song can turn dark without moving anywhere.