A substitution used a lot in jazz, the tritone substitution is where we follow a dominant chord with a similar dominant chord which has a root note three tones higher than the original root. This means that the chord will be a flat fifth higher than the original. This interval is referred to as a tritone, also known as 'Diabolus in Musica' in early music (see 'Intervals').
The Tritone substitution is used when we have a dominant chord followed by another dominant chord one fourth higher or one semitone lower, for example:
We can also replace the A7 chord with it's minor (II) substitution - Em7 (see 'V Substitutions'), which will give us a chromatic line through the bass line:
