Substitution RULE #3: Tritone Substitution. This rule states that the "tritone" (an interval of three wholesteps) can replace its parent dominant chord. For those who are classically in-tuned this is called a "secondary" dominant or in this case a "V" of "V" chord or V/V. After you find the V/V chord then you will have to lower (flat) the secondary dominant. A tritone is found in every dominant seventh chord and is marked by instability. The tritone defines the two dominant chords. Using the G7 as an example. Its "b5" is a Db7 chord. The tritone B-F is present in both chords. In saying all this, the rule simply states that the "b5" dominant chord of the dominant chord can be used in place of the 5 dominant 7th chord.