“Modulation” in music means “change of key”. A modulation can happen with or without a change of key signature, and in fact, most modulations do not include a change of key signature.
Changes of key can be either momentary (just a beat or two) or sustained (continuing for a phrase or more). When a key change is momentary, we say the music is “passing through” a new key. When the key change is prolonged, we say the music has “modulated”. With a modulation we have a sense of a new tonic/keynote, whereas when the music is passing through a key (or keys) the keynote changes too quickly for the ear to settle on it.
Modulation happens when a note/chord which is outside of the current key is used as an important note. If a chromatic note is used in just a decorative way (e.g. passing note), it does not cause a change of key. We will learn more about decoration later in the course.
Most often, a modulation occurs with the chord progression V-I (or V7-I) of the new key. The point of modulation is the place where the “new” note occurs. The music will then continue in the new key and the phrase will normally end with a cadence in the new key.
- “New note” from new key in chord V or V7
- Chord I from new key
- Continue in new key
Let’s say we have a piece in C major, and we want to modulate to A minor (the relative minor key).
If we look at the C major scale and A minor harmonic scale (the harmonic minor is usually used for harmony) we can see the only difference is the note G#. We need to use this note, and the chord that this note belongs to, to achieve a modulation. G# occurs in chord V, or V7 in the key of A minor.
We also need to remember that because G# is the leading note in A minor, we should observe the correct voice leading for leading notes and have it rise by step to the tonic A in V-i.
In this tune, the modulation starts with the G# at the end of bar 4. G# moves to A (tonic chord i), and the music continues with chords from A minor. It finishes with a perfect cadence in the same key.
Notice that in bar 4, chord V of C major moves to chord V of Am (bar 4). The progression V-V or V-V7 of another key can happen when the music is modulating.
Using the same beginning, let’s try a different modulation, this time to F major. Comparing the scales of C major and F major, the only difference is Bb. This note is the 7th in V7, so we will need to remember the voice leading of 7ths, and let the Bb fall to A in chord I of F major.