The most common substitution, used extensively in jazz is the “tritone” substitution. This is where the root of a dominant 7th chord is replaced or substituted with the root a tritone lower (or higher, they are both the same note). This is often achieved by using a b5 chord which is a dominant 7th chord with the 5th note flattened.
For example here is a diagram of the chords C7 and Gb7. The arrows indicate the 5th note which is flattened to provide the tritone. You can see that the final two chords of C7b5 and Gb7b5 are the same.
It is therefore possible to use a Gb7 chord in the place of a C7 chord. But why is this so important in music? It all comes down to “voice leading”. Let us have a look at a very common chord progression on a SeeChord chart:
Circle of fifths chord progressions
This chart shows a strong progressions of falling fifths, all using the dominant 7th chord. This progression can be found at the end of the jazz blues example on the BLUES page.
Voice leading refers to the movement of an individual line through a piece of music. It is most clearly seen in music that has a set number of distinct parts such as a choir containing soprano, alto tenor and bass parts all singing at the same time. It was J.S.Bach who formalized many of the “rules” of writing in this format as he wrote hundreds of “chorales” which were short sacred pieces for four parts.
It is the bass part that we are most interested in here. The reason is that bass lines can shape the sound of a piece of music enormously. Our ears seem to be drawn to the lowest note being played and use that note as a reference point for the other notes being played. Bass lines that move in pleasing ways therefore seem to produce pleasing music! The most pleasing bass line of all would seem to be the descending bass line.
That brings us back to the tritone substitution. The following chart shows what effect substituting alternate chords of a descending fifths progression will have:
It looks quite random on the chart, but this pattern represents a “chromatic” descending bass line. Have a look at the chords in the “chord row”. E-Eb-D-Db-C. Suddenly, by clever use of the tritone substitution, we have created a piece with one of the strongest chord progressions as well as one of the strongest bass lines. It is no wonder that this technique is so widely used in music.