Modulation
Modulation is a change of key within a piece of music, where a new note takes over as the tonic and a cadence in the new key confirms it.
What does a modulation look like?
The smooth way is through a pivot chord, a chord that belongs to both keys. Going from C major to G major:
- Play in C major: C, Am. Here A minor is vi.
- Treat that same A minor as ii of G major, and follow it with D7, the dominant of G.
- Resolve D7 to G and stay there.
Nothing outside either key sounds at the moment of the change. Only afterwards does the ear realise home has moved.
The blunt way skips the pivot. Pop songs often lift the last chorus straight up a semitone or a whole step, as Whitney Houston’s “I Will Always Love You” does near its end.
How do you hear a modulation?
Two signs. A new accidental keeps coming back: an F♯ that turns up again and again in a piece in C major suggests G major. And phrases stop ending where they used to: cadences start landing on the new note, and it begins to sound like home.
Most modulations go to a closely related key, one whose key signature differs by at most one sharp or flat. From C major that usually means G major, F major or A minor.
What is it confused with?
Tonicisation. A secondary dominant such as D7 in C major points at G for one chord and then the music returns home. Without a cadence that settles in G, the key has not changed.
Transposition is different again: moving a whole song into another key, for example to suit a singer. Nothing inside the song changes key; all of it moves together.
Try it: the modulation lesson ends with a drill where every progression changes key, and secondary dominants shows the brief pull that is not a modulation.