As long as I started this thread, I might as well politely offer my disagreement that you can just throw in a few more chords and folk music becomes jazz. To me, jazz is as much a feeling as a series of particular chords. And, most traditional folk music at least IS based on three chords. That doesn't mean that you can't add chords to your own taste. For me, the great appeal of folk music has always been its simplicity. I've heard a lot of string bands try to do jazz and for the most part, they sound like string bands trying to do jazz. A couple of weeks ago, I heard a woman playing a piano accompaniment from sheet music for a black gospel song, and it really sounded weird. She was playing the notes on the page correctly, but it didn't have any feeling to it. I've heard technically proficient guitar players do ragtime piano pieces that could put you to sleep. They got every note right, but the rhythm was stiff.

But, that's just my opinion...

Jerry