Seminal American blues singer and songwriter Muddy Waters. Keystone/Getty Images hide caption

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Earl Scruggs performs at the Stagecoach Music Festival in 2007. Karl Walter/Getty Images hide caption

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Woody Guthrie. Robin Carson/Courtesy of the Woody Guthrie Archives hide caption

itoggle caption Robin Carson/Courtesy of the Woody Guthrie Archives

Al Green, from the cover of his Greatest Hits album, first released in 1975. Courtesy of the artist hide caption

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Kurt Cobain performs with Nirvana at a taping of the television program 'MTV Unplugged' in New York City, Novemeber 18, 1993. Frank Micelotta/Getty Images hide caption

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