EJ17s offer heavier tension resulting in a bolder, more resonant acoustic tone with higher resistance for heavy strumming and flat-picking. Buy 3 sets and save! Phosphor Bronze was introduced to string making by D'Addario in 1974 and has become synonymous with warm, bright, and well balanced acoustic tone. D'Addario Phosphor Bronze strings are precision wound with corrosion resistant phosphor bronze onto a carefully drawn, hexagonally shaped, high carbon steel core. The result is long lasting, bright sounding tone with excellent intonation. Buy 3 sets and save with this environmentally friendly, corrosion resistant 3-pack D'Addario's most popular choice for heavy strumming, flat-picking and resonant, projecting tone Preferred for its warm, bright, and well balanced acoustic tone Made in the U.S.A. for the highest quality and performance String Gauges: Plain Steel .013, .017, Phosphor Bronze Wound .026, .035, .045, .056