When you modulate from one key to another, your composition changes color. Even if all you do is repeat the same melodies and chords in the new key, those melodies and chords sound different in comparison to their original presentation. Not only that, a melody being played a half-step (or a whole step or a third, a fourth, or a fifth) higher triggers an emotional response from the listener. Key changes are a great tool, and very easy to do. Here’s how.
Changing keys is easy. Physically, all you have to do is insert a new key signature in that measure where you want the key to change. If the key change takes place at the start of a new staff, the old key signature should be cancelled out at the end of the previous staff. For example, if you move from the key of F (with one flat) to the key of G (with one sharp), you use a natural sign to cancel out the flat of the old key and a sharp to introduce the new key.
Harmonically, changing key signatures is a little more involved. Ideally, you want to modulate to a key that is somehow related to the previous key. You should familiarize yourself with several common types of modulation.
This is a very simple modulation, moving your entire composition just a tad higher. This type of modulation is common in popular music and is effected simply by moving up a half-step to the new I chord—no connecting cadences necessary.
This is similar to the half-step modulation, but a little more dramatic. Again, there is no connecting cadence before the modulation; just step up to the new I chord.
Technically, this modulation is down a perfect fifth, although the melody is often transposed higher (up a fourth) rather than lower (down a fifth). This type of modulation sounds very natural because the old key functions as the dominant of the new key. For example, if you change from F to B♭, F is the dominant of B♭.
You typically make this modulation via a slight connecting cadence; all you have to do is turn the tonic chord (I) of the original key into a dominant seventh chord. This dominant seventh then cadences naturally to the tonic of the new key. For example, if you’re modulating from C to F, turn the C chord into a C7 chord, and then lead from that into the F major chord—the I of the new key.
Another way to determine which key to move to is to move to a key that shares one or more chords in common with the original key. That shared chord can then serve as the pivot point for the modulation.
Let’s look at an example, starting in the key of C major. The D minor (ii) chord also exists in several other keys, including the key of F major, where it serves as the vi chord. You can modulate from C to F by holding the D minor chord and then using it as the vi chord in the new key. From your chord-leading rules, you know that the vi chord easily leads to the IV chord, which, in the key of F, is a B♭ major chord. So you hold the D minor chord and then move to a B♭ major chord (and then to the next appropriate chord in the new key). Because B♭ major is definitely not a chord in the original key of C, the listener is immediately made aware of the modulation.
There’s no rule that says you have to follow these modulation guidelines; it’s perfectly acceptable to change from any one key to any other key, with no warning or connecting cadence necessary. For example, you can modulate from the key of C to the key of A simply by moving from one tonic chord to the other. This is called an abrupt or direct modulation and has a very unsettling affect. It definitely calls a good deal of attention to itself.
Now that you know how to do modulate to a new key, you can insert a key change into your composition and take it to new heights! Happy composing!
From The Complete Idiot’s Guide to Music Composition by Michael Miller