D'Addario 10-Pack EJ17 Phosphor Bronze, Medium, 13-56EJ17s offer heavier tension resulting in a bolder, more resonant acoustic tone with higher resistance for heavy strumming and flat picking. 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