Here's some improvisation over a home made midi track of this (slightly doctored) famous chord progression. In dropped D tuning, playing through a DigiTech RP100 using a heavy brass slide. Visit the site ( http://www.bottleneckguitar.com ) for more info on slide in standard and dropped D.I don't ever think scales when improvising like this, I follow the chords. . The trick is to be able to see the whole fretboard as 'the chord', which is what my book/DVD package PlaneTalk teaches. You can find out more about it at http://www.thatllteachyou.com ... from then on, it's like joining the dots.