25
G
G
A
B C
D
E
F
C
D
E
F
G
A
B C
D<
E<
F G<
A<
B< C<
D<
A<
B< C<
{
{
Tritone Interval
Tritone Interval
Gdom7
(original)
G, B, D, F
C major
(tonic)
D<dom7
(substitute)
D<, F, A<, Cb
Another reason this substitution works, even though it is not diatonic, is that the
root of the substitute chord which creates harmonic tension descends one half-
step to the root of the tonic, creating a descending chromatic bass line. The tri-
tone interval of the original dominant chord and its flatted-fifth substitute allow the
tension created by these chords to be released by the tonic chord.
Another interesting characteristic about the tritone interval is that two tritone
intervals added together produce an octive. As in the example shown above,
when we go up the scale three whole steps from F (the second note of D<dom7),
we land on C< (the fourth note of D<dom7). When we go down three steps from
F, we land on C< an octave lower than the fourth note of the chord. Since C< and
B are enharmonic equivalent notes, you can see that D<dom7 and Gdom7 share
the same tritone interval from F to B and from B to F.
Flat-Five Substitution (Tri-tone)
Another kind of chord substitution which is often used in pop and jazz music is
know as
flat-five substitution.
This substitution is made by replacing a dominant
chord with a new dominant chord whose root is a diminished fifth interval above
the original chord’s root. For example, in the key of C major, a Gdom7 (G7)
could be replaced with a D<dom7 (D<7). (G to D< is a diminished fifth interval.)
This is not a diatonic substitution because D< does not belong to C major.
However, the two chords do share a
tritone interval.
A tritone interval is actually a diminished fifth (or augmented fourth) interval. This
interval is made up of three whole steps, thus the name
tritone
. This substitution
works because these two chords resolve to the tonic chord in similar ways.
In the key of C the dominant chord is Gdom7. The flatted fifth of Gdom7 is
D<dom7, which becomes the substitute chord. The example below shows that
the second and fourth notes of these two dominant chords resolve to the first
and second notes of the tonic chord by descending or ascending one half-step.
