b5 subs

Take a 7th chord, like G7: G-B-D-F

The tension in that chord comes from the interval between the third and seventh, B and F. Those two notes are a tritone apart, a dissonant interval that needs to resolve.

Now look at the 7th chord a flatted fifth higher - that would be Db7:

Db-F-Ab-Cb

The dissonance comes from the same two notes, the third and seventh... and since Cb is enharmonic to B, they're exactly the same two notes!

That's how it works. The dissonant interval is identical, but the consonant surroundings are changed.

I assume you are referring to tritone substitution?
If so, that only applies to dominant chords:

C7 / F#7

D7 / Ab7

You can indeed substitute one with the other if the melody allows this. You could even put the II chord of the substitution key in front of it (something Brad Mehldau is very fond of in his solos, use with moderation though):

C7 | F | -> C#-7 F#7 | F |

Brad Mehldau does this on the fly without the bass player following him creating an "outside" sound, even sometimes anticipating the substitute II with a halfstep and without resolution to the V . This is in fact the "magical" Brad Mehldau sound many piano players are searching for (it's just one of them though!!).

Niliov

hehe, Noteboat and me answering at the same time, lol!!

ya that makes sense thanks

Thought I'd add this:

for those who can't quite figure it out immediately it is very easy to see on the guitarneck:

Let's take G7 and play it like this:
3X34XX with these fingerings: 1X23

now play Db7 (its tritone substitute) like this:
X434XX with these fingerings: X213

If you try this you can see the chords are very similar!

It gets even more interesting when you add extensions to the chords, for example:
G7(13) like this: 3X345X with these fingerings: 1X234

and now Db7(b10): X4345X with these fingerings: X2134

You might notice only the bass note changes, but that changes the complete sound of the chord.

One more:
G7(#11): 3X342X fingerings: 2X341
and
Db7: X4342X fingerings: X3241

And there we have a standard guitar voicing for Db7 but a rather exotic one for G7!

Niliov