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I always considered The Rooftop Singers and Kingston Trio as the cool side of the American Folk Revival. Peter, Paul and Mary as the Pop side. They (PP&M) played Nylon string guitars for Pete's sake. There is no nylon in Folk. You won't find a nylon guitar playing in Country Blues or Olde Timey or traditional Appalachian Folk music. I was undecided about Bob, and bit suspicious if he was just jumping on the bandwagon with Folk. I seen a doc about him and I guess he was legit. He came across as a music lover.
Walk Right In is amazing, you have to hear it in full to get the 12 string work, in some small way it changed the face of music. (Erik Darling from The Rooftop Singers is one of my music heroes.)