In music, substitute dominant chords are also known as Sub-V (pronounced Sub Five) chords because they originate from a reharmonisation of the original dominant chord. For example, in the key of C Major, the V7 of C Major is G7. The notes in G7 are G,B,D and F.
The guide tones (which are also the tritone interval) is B and F. The Sub-V chord is found by finding a new root for the original G7 chord, looking at a diminished fifth above, hence the note Db. By keeping the original B and F notes and changing the root note to Db, you get a new dominant chord which is the Db7. This also creates a nice chromatic motion downwards a half-step resolving into the C Major chord. Hence, the reason why bebop players called this the Sub-V reharmonisation. This can be done on any dominant chord that originally intended to resolve a perfect fifth down, creating a new resolution a half-step down.
