D'Addario B13556BT Nickel Bronze Acoustic Guitar Strings, Balanced Tension Medium, 13.5-56 Nickel Bronze strings are the premium uncoated acoustic strings by D?Addario. Featuring an innovative combination of nickel-plated phosphor bronze wrapped onto a high carbon NY steel core, these strings bring out the unique tonal characteristics of any guitar, allowing its natural voice to truly shine. NB13556BT delivers a balanced medium tension string set resulting in a bolder, more resonant acoustic tone with ideal resistance for powerful strumming and flat-picking. String Gauges: Plain Steel .0135, .018, Nickel Bronze Wound .024, .032, .042, .056