Affectionately known as ISB, they were a psychedelic folk band originating in Scotland (variously Robin Williamson, Clive Palmer, Mike Heron and Licorice McKechnie), who built a range of dedicated followers from 1966 to the mid 1970s.
They were musical pioneers, admired by Bob Dylan, playing Newport Festival on a bill with Joni Mitchell and Leonard Cohen, integrating traditional music/folk forms with ever more instruments from the world music catalogue.
I is also for instruments
Of which there are almost enough to make an A-Z in their own right. Used in folk music besides guitars are accordian; autoharp; bagpipes; banjo; concertina; cymbals; dulcimer, bowed dulcimer and hammered dulcimer; African djembe; drum; fiddle, fiddle/violin and electric violin; flute; handclapping; harp; harmonica; hurdy-gurdy; jew’s or jaw harp; mandolin; pennywhistle; piano; ring flute; sitar, tambourine and washtub base. Not to forget the voice, from harmony to plainsong.
Listen (and watch): The Incredible String Band, 1968, The Half-Remarkable Question, playing some of their range of instruments