When a piece of music moves from one key to another the process is called modulation. There are, however, modulations which last only briefly, where the new key may only be suggested, and those modulations where a new key is established for longer periods, usually by a cadence in the new key.
Several techniques are commonly used by composers to effect both types of modulation.
The first is through the use of a Pivot chord, or Pivot chords, which are chords common to both keys: the one you are modulating from and the one you are modulating to. If we look at the harmonised keys of C major and G major
we see that there are four chords which are common to both: C major, E minor, G major and A minor. Because these two keys are closely related, G is the dominant of C, they have more chords in common than two unrelated keys (other chords can also become available through the use of mixture). By using a pivot chord, or chords, we may play a pivot chord in one key and continue our progression in a new key. In the following example the C major chord is heard as the tonic in C major (through a I – V – I progression in that key). The C major chord is then left as chord IV in G major where it progresses towards a perfect cadence (ii – V – I) suggesting G as the new key centre.
Notice that the key of G major is not neccesarily present until the F sharp has been introduced, and the cadence has sounded. This is because the first three chords in the key of G major also belong to C major: C major, G major and A minor; the use of the C to G progression, which has just been played in the key of C, also means the move to G is not fully felt. Even with a perfect cadence a new key may not become fully established until further activity confirms this key. Consider the following
In this example the ii – V – I cadential progression from above is treated as a secondary ii – V progression which embellishes chord V in C major. The secondary nature of the progression can be confirmed by the immediate reintroduction of the F natural and a perfect cadence in C major.
In the following example
G major has been further established by the melodic line and another cadential progression in G. However, because of the many common chords in this progression, further G major activity is probably needed to fully establish the new key centre.
As you can see and hear in these examples, the move to a new key may not be felt even after a cadence in the new key has been played. Often a change of key may only be suggested by a temporary introduction of an accidental, such as the F sharp in the above examples, or, the composer may simply be using secondary chords for added colour.
Another common means of effecting modulation is through the use of a common note, or notes. Like the pivot chord, common notes are the same note/s found in two different chords: one from the first key and another from a second key.
In the following example
the note C is found in both the C major chord and the A flat major chord. In this example, because of the third relationship between the roots of the C major and A flat major chords, the modulation sounds ‘smooth’. Whereas in the following example
the held E whole note effects the modulation between C major and C sharp minor; the semitone relationship between the two tonic chords of these keys means the modulation is a little more abrupt.
Sometimes two common notes are used to effect a modulation
Here the notes A and C are held from chord ii in G major to become part of chord V of B flat major.
Using fifth relations between chords, based on the circle of fifths, is another common means of modulating. In the following example
the cycle of fifths begins in the key of E minor, starting with the subdominant seventh chord. As the cycle continues, chord VI in E minor is chromatically inflected becoming a C sharp dominant seventh chord, instead of the expected submediant chord in E minor (C major); this inflected dominant moves the music into the key of F sharp minor. This technique can also be used with other chords from the cycle of fifths: In the following example
the G mediant seventh chord in E minor is changed to a G dominant seventh chord which moves the music into the key of C major.