Roots-music royalty convenes at the Beacon when alt-country godmother Emmylou Harris hits the stage with her longtime compadre, singer-songwriter Rodney Crowell, to showcase material from their handsomely twangy joint CD, Old Yellow Moon, issued on Nonesuch. Opening is English folk-rock legend and everyman guitar hero Richard Thompson, who leads his current combo in songs from his 14th studio album, Electric, recently issued on New West.