Changing to the key a fifth below the current key is very easy, and can be useful sometimes. If the last chord you play in the old key is a I chord (and I normally will be), simply change it to a dominant chord by adding a minor 7th. This chord will automatically be the V7 in the new key which naturally resolves to the I chord.
Here is the simple formula:
|Old Key||New Key|
|I – I7||I|
Here is an actual example. It is simple, but it works.
Note that in this example, the first chord has a major 7th in it (B natural). To change the chord to dominant, you simply have to lower the B to Bb.
I want to take a moment and talk about the function of the dominant chord. The reason that a dominant V chord is a critical part of many modulations is because of its strong desire to resolve. When it does resolve, it almost always resolves down a fifth. It is without doubt the smoothest chord resolution in Western music, and is the way almost every song in Christian music ends.
Interestingly, while our ears are accustomed to the sound of a dominant V chord, it is actually very dissonant and unstable. The dissonance comes from an interval in the chord called a tri tone. Two notes that are 3 whole steps apart create a tri tone interval. In the example above, the triton interval is created by the E and Bb. The two notes that make up a tritone interval usually resolve in contrary motion–in this case, the E resolves up to F and the Bb resolves down to A.
Over time, I will talk much more about dominant chords. They are used extensively in modern music including our church music. The V chord is only one of them. You will also see dominant I, II, III, VI, and VII chords, which are called secondary dominants. They all function in essentially the same way, normally resolving down a fifth.
If the three preceding paragraphs do not make sense to you, just ignore them for the time being. You can certainly use this simple modulation without knowing why it works. One anology that I find helpful is that of magnets. If you try to push together the wrong ends of two magnets, it is possible to do, but a lot of tension is created. As soon as you release your pressure, the magnets cannot help but change position and join themselves in another way. Chords like the dominant chord include notes that create tension and and those notes are
practically itching to move somewhere else. People that play by ear well do not have to try to figure out where they want to resolve–they just instinctively know where to go next.
Practice this modulation formula in every key. Identify the tritone interval in each dominant chord and note how it resolves.