D'Addario EJ17 Phosphor Bronze 13-56 Medium Acoustic Guitar Strings - 3 Pack D'Addario Phosphor Bronze Acoustic Guitar strings are acclaimed worldwide for their great, long-lasting tone. Phosphor Bronze was introduced to string making by D'Addario in 1974 and has become synonymous with warm acoustic tone. String Gauges: Plain Steel .013, .017, Phosphor Bronze Wound .026, .035, .045, .056