What do Taylor Swift and Neil Young have in common? Not much, perhaps, but both play in a style that can easily be traced back to the same troubadour family, the Carters. At a time when the country was bursting with different musical traditions, A.P. Carter and his kin took the rich harmonies of church music and combined them with the lilting rhythms of the guitar and autoharp. The result was an infectious blend that shot into the veins of American popular music and has been there ever