I was just practicing and my train of thought went from making a rhythm track for myself of St. Thomas to how trading 4's and trading 8's works between two melodic instruments. I understand how it works with drummers -- the entire form keeps moving. But if, say, the sax player and the trumpeter in a quintet were to trade 4's while keeping in the vein of playing-what-the-other-plays-with-a-tag (eg, what I almost always hear when people do this,) how does this affect the form of the song? Do you just pick a section of the tune -- say, the bridge when trading 8's on a standard -- and repeat those until the two are done? What's the deal? Sure, there's common notes across chords, but I hear some guys trading back and forth for ages and I've never seen a non-modal tune where keeping the structure moving as normal with that is even remotely feasible.