Nickel on Steel. RotoSound's highly secret core to cover ratios used in the making offers such brilliant sustain and tonal nuances few others can equal. The longer European eye adds extra strength at the ball-end. RotoSound's unsurpassed quality and constant care to detail throughout every stage of the production process culminates in a string equally geared to the professional and hobbyist alike. All Rotosound music strings are manufactured in England and proudly served up at Twin Town Guitars! • MFSKU R10 • UPC 686194000509