Matching a scale or mode to a chord is not that difficult. Even beginners are able, after a relatively brief period of time, to match up say a G major scale with the Gmaj chord or play a minor pentatonic scale over a G7 or whatever. The problem is that we get trapped into thinking that we are obligated to play certain scales with certain chords. That's too bad because in so doing people miss out on a lot of colorful ideas.
Take a look at the diagram below. Obviously, if you're playing in the key of F it makes 'sense' to use the F Ionian mode and it isn't that great of a leap to realize that you can play the D natural minor/Aeolian mode 'on top' of the F maj chord as well. The D natural minor and the F major scale possess the same notes. Many chords and scales are interchangeable like this. But what is missed is that you can apply the idea of interchange in a parallel fashion. Where the D Aeolian is 'derived' from the F major scale we can take a parallel approach and try the F Lydian with the F maj chord or the F Aeolian etc. 'But it will sound weird!' Yeah, but that's the point. Enter the realm of 'bitonality.'
Unless we're only dealing with the bitonality of playing a G blues scale over a G7 chord this isn't going to work so well in a campfire setting or in your local church (and please don't try this in Branson or you'll be thrown out of town). But if you want to get 'out there' this is a good way to do it. Jazzers have been doing this a long time with hybrid scales and tetrachords and in the pitch axis perspective where you could cycle through a cycle of 5ths to reach a target. This approach will work really well in harmonic environments that remain relatively 'static' for extended measures. A good example of this is Scofield's recent recordings where he works through long vamps. Playing the same old diatonic lines for, say, 32 measures can get pretty boring. So taking this 'liquid' approach is a good way of implying harmonic movement where there is none in the harmonic progression. This can be a very propulsive strategy of improvising.
