Modulation means changing the key partway through a song. Raising the final chorus by a semitone to lift it, changing the scenery in an interlude, creating a sense of elevation at the end – it is used when you want to move the development of a song in a big way.
What changes when you modulate?
When the key changes, so does the note you feel as the center and the set of scale notes and chords that come naturally. Move from C major to D major and the same melodic shape sits in a different register and feels differently bright.
Going up a semitone is one kind of modulation
The familiar pop move of raising the last chorus by a semitone is also a modulation. Because it rises only a little from the previous key, the melody can stay the same and still sound like it gained energy. In vocal music, though, the register goes up, so check that it is still singable.
Connecting it naturally
Placing a chord shared by both keys around the change, or sounding the dominant chord of the new key first, tends to make the move sound natural. When you build a chord progression, compare jumping straight into the new key with taking it in stages.
Things to watch in a DAW
With MIDI, edit the notes and chords from the modulation point onward to fit the new key. If you are using audio loops, check that pitch shifting has not made the sound quality or the timing unnatural. For material labeled with a key, choosing one that matches the key after the modulation is the safe approach.
Distinguishing it from a similar term
In music production, “modulation” is used to mean changing a sound over time, which is a different term from key modulation. This page deals with moving the key as part of the structure of a song.
