There is a trick you can use to add tonal variety to the music you play. It involves substituting any dominant chord (like G7, A9, C7#9) with another dominant chord with a different root. Not just any substituted root will work, however. To make this trick work, and by that I mean sound good, you need to follow a specific rule.
The rule to making what is called a tri-tone chord substitution is, the substituting chord needs to be a dominant chord whose root is a diminished fifth above (or below) the root of your original chord.
This means if you see a G7 chord you can substitute it with a chord whose root is a diminished fifth higher than the G. In this case, that note would be a Db (D-flat). It’s called a tri-tone because the substituting chord is three whole steps higher (or lower) than the original chord. As it turns out, whether you go up or down three whole steps from any note the resulting note is the same. So, from G you’ll land on Db either way you go.
This chord substitution trick works so well because the resulting chord possesses two important notes of the original chord. These two notes are crucial to creating the right harmonic tension. In the key of C Major the G7 chord often precedes a C Major chord to produce a comfortable harmonic pattern that resolves the tension created by the G7. The movement from the tri-tone interval of the G7 to the major 3rd interval created by the C and E of the C chord produces a nice resolution. Even though the chord Db7 does not belong to the key of C Major, this chord resolves in much the same way. Both chords have a B and F (Cb=B). The diagram below shows how the notes of G7 and Db7 relate to Cmaj7.
If none of this makes any sense to you, don’t worry. I just wanted to mention the theory behind it for those who want to know. You don’t need to know any music theory if you can remember one thing. In fact, you don’t even need to remember anything if you can draw a circle and put some letters around it.
As it turns out, the diagram know as the Circle of Fifths shows you exactly what chord root to use without any thought at all. All you need to do in order to find a chord to substitute for a dominant chord is to chose the note opposite the circle from the root of your original chord.
To the left is an example showing the root of the chord you would use if you came across a G7 chord and wanted to play the appropriate substitution.In every case your substituting chords is on the opposite side of the circle. For instance when you see a C7 chord in your music, play the Gb7 instead. When you see a D7, play an Ab7.
Chord substitutions are not just for creating different sounds in you music. They are also good for when you encounter a chord you don’t know. If you don’t know the original chord, the chord that is opposite it on the circle may be familiar to you. Also, if you need to play many chords at a fast tempo, you may want to choose chords that are easy to play and close together. This substitution trick can help there too.