This American icon started out as a blues troubadour with singles like “Packin Trunk” from the ‘30s. But he made his true mark as a folk innovator with a forceful voice and strumming technique that looked ahead to rock ‘n' roll: Tunes like “Rock Island Line” developed rollicking textures that would influence skiffle acts, and tracks such as “Jim Crow Blues” were early examples of folk music topicality. The Library of Congress sides from the ‘40s give us a window into his performance style—driving, sensitive and humorous. Without him, popular music as we know it wouldn't exist.