When Nuno Bettencourt and Dean Markley designed these strings, they insisted on a super high grade Nickle Plated Steel outer wrap. This outer wrap uses special (and secret) Nuno Bettencourt core to wrap ratios around a brilliant and sturdy tinned mandolin wire. This carefully crafted string produces a bright, fat, powerful sound, which holds its great tone for that exceptionally long time you've come to expect from Dean Markley strings.