Ken Burns and his collaborators for the upcoming docu-series Country Music show a clear and understandable affinity for Johnny Cash. The Man in Black and his extended circle -- the Carter Family, his former bandmate and son-in-law Marty Stuart and others -- play key roles in a narrative that explains the very fabric of American society through the winding history of an enduring musical genre.

It’s no surprise, then, that a preview clip for the series shows Rosanne Cash explaining the origins of one of her dad’s most beloved songs, “I Walk the Line.” Per the younger Cash, the song served as a promise to her mother, Johnny Cash's first wife, Vivian Liberto, to avoid the carnal temptations of the road.

“The song came from my mother’s fear. You know, ‘You’re going out on the road, and these girls are coming up to you,'” Rosanne Cash says. “He wrote "I Walk the Line" [as a way of saying], 'I’m going to stay true to you.'”

Cash never shot a man in Reno just to watch him die, but at least one of his all-time classics came right from his oftentimes-turbulent life.

Country Music, an eight-part documentary, debuts on Sept. 15 on PBS. The show's two-hour episodes will air in consecutive Sunday-through-Wednesday blocks (Sept. 15-18 and Sept. 22-25).

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