Originally Posted by: Fret spiderthats cool guys.

just checkin, ,mark, you are saying that you have your normally the 7th chord of c ionion would be (i dont know the name) root(B), minor3 (D), diminishd 5th (F), minor 7th (A).

That's a Bmb5, also sometimes called a B half-diminished.

but we play the diminished scale with the bb7 (Ab). so this clashes with the chord and the key. but it doesnt clash so much that tonality is lost. so tension is built up.

Kinda, but not exactly. I guess you could do that, but I was substituting the V chord (G7) for the full B diminished 7 chord. A G7 has the notes G B D F, while a Bdim7 chord has the notes B D F Ab. So they kinda fit together.

so when u use this do you try and resolve the Ab to an A as u play or do u just plow ahead with a diminished scale, letting the b7 and bb7 clash. is it a bit like playing a minor third over a 7th chord in a 12 bar blues?

You just plow ahead! If you played D B F A you'd be outlining another chord altogether. It is a bit like playing the blues scale over a major blues.