Another common method of substitution is to use a chord whose root is a tritone (flattened 5th or augmented 4th) above that of the original chord. This is known as tritone substitution. As with other methods of substitution, this works because there are notes common to both chords. Shown below are the chords G7 and 07. As you can see, the note F occurs in both chords. The note B is enharmonically the same as O which is the 7th of 0>7# so this note is also in both chords.
In the second half of the following example, Db7 is substituted for G7, creating a II bII I progression in the key of C, instead of a standard II V I. Notice how the notes Db and Ab in the Db7 chord give the progression a different flavor to the II V I using a G7 chord.
Tritone substitutions are most commonly used with dominant chords, but can be used with any chord type and on any degree of a scale. In some cases, every chord except for I can be a trrtone substitution. The next example uses a I VI II V progression, followed by a second version where bIII is substituted for VI, bVI is substituted for II, and bII is substituted for V. This is then followed by two more variations. This type of substitution throughout a progression is sometimes called backcyding. These are only a few of the many possibilities. Experiment and come up with some of your own.
A good way to practice substitutions is to learn a basic progression and then try out various substitution possibilities with some or all of the chords. The following example is based on the chord changes to George Gershwin's "I Got Rhythm", and is commonly referred to as "Rhythm Changes" It has been recorded on the CD without a guitar part so you can jam along with the rhythm section. Play it along with the CD until you have the form memorized.
Here is another version showing some possible substitutions. With any chord progression there are many substitution possibilities. The trick is knowing which ones to use in a particular musical context, and also when not to use substitutions. This knowledge comes from knowing the sounds of a particular idiom and is best developed by a great deal of listening and by frequently playing with other musicians.