One of the most popular jazz chord substitution is the tritone substitution sometimes referred to as the dominant chord substitution. The most used is to replace the V with a dominant 7th chord whose root is a tritone below. Example with G7 (V), which can be replaced by Db7 a tritone (Three whole-steps) away. Thus giving two chords that have two notes in common. The 7th of G7 (F) is the third of Db7 and the third of G7 (B) is the seventh of Db7. The inversion of the 3rds and the 7ths between the original dominant chord (V) and the substituted dominant chord (bII7) is the main feature of the tritone substitution.