Reharmonizing the V7 with the SubV7

One of the most-often-used re-harmonization of the V7 is done by replacing that chord with the subV7.

Which chord is the subV7
The subV7 is a dominant chord a tritone away from the V7 (also a half step above the tonic or I)

Why does the subV7 work as a substitute of the V7?
There are two main reasons why this chord is such a perfect substitute for the V7

• The guide tones (3 & 7) are the same in both functions.
  The 3 & 7 in the subV7 are the 7 & 3 in V7. The guide tones are the notes that give the chord its characteristic sound. In dominant chords, the guide tone (3,7) are a tritone apart. The tritone is an interval with lots of tension, [it used to be called “The devil’s interval” back in the middle ages] , which wants to resolve by moving each note half a step in opposite directions, thus turning into a major 3rd (when resolving inward) or a minor 6th (when resolving outward)
  Example: In the key of C • G7 is the V7 • 3rd is B • 7th is F (since it is a flat seventh in dominants). B-F is a tritone. It wants to resolve either: inward to C-E (major 3rd) or, outward to Bb-Gb (minor 6th). A tritone apart from G is Db which would be the root of the subV7 in the key of C, and the guide tones in Db are F (the 3rd) and C flat (the flat 7th, which is B). Thus, we can resolve those two notes, as if we were in G7, to E and C, the tritone then resolves as expected in the key of C from Db7.
• The root of the subV7 is a half step above the I (tonic)
  The bass-line for a V7-I progression moves down a perfect fifth (which is a very powerful bass movement).
  When substituting the V7 with the subV7, the bass-line for the subV7-I is moving a half step down (which is as strong as the perfect fifth). Substituting the V7 with the subV7 gives us a strong bass-line that resolves tension well.

Here are the two examples using Mapping Tonal Harmony Pro 5 to view the different paths taken when using the V7 and/or the subV7 in the progression IIm7 – V7 – I (2-5-1)