D'Addario NB1356 Nickel Bronze Acoustic Guitar Strings, Medium, 13-56 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. NB1356 offer heavier tension resulting in a bolder, more resonant acoustic tone with higher resistance for heavy strumming and flat-picking. String Gauges: Plain Steel .013, .017, Nickel Bronze Wound .026, .035, .045, .056