...resulted from the migration of American blacks from the South. Delta Blues, with the addition of electric guitars and drums - this may be part of the origin of the term rhythm and blues. Its most famous exemplar was probably Muddy Waters (who wrote "Rolling Stone"), but don't forget Willie Dixon, Otis Spann... Many of their 40's/50's tunes later ended up in the repertoires of aspiring rock bands, sometimes inspiring lawsuits when people had brain cramp with respect to copyright law.