A Sub Five (V) chord is a substitute chord for the 5 (V) chord in a progression which happens to be a dominant chord. In the case of blues that uses most dom 7 chords, you can use the sub 5 (V) chords replacing all the original chords if you chose too. This method is used a lot in the style of jazz.Example. In a A Blues progression the DOM 7 chords would be
A7, D7 ,E7
You can replace these chords with their sub fives such as
[A7 = Eb7] [D7 = Ab7] [E7 = Bb7]
Note the sub 5 (V) shares the same tritone as the original chord. For example the A7 chord tritone contains a G and a C# so does the Eb7 chord. Cool eh! Below is the 12 bar blues form using the sub 5 (V) concept. The sub fives (V) are highlighted in blue.
See Attachment. Have fun!