XS Phosphor Bronze strings have the longest life of any D’Addario acoustic guitar string. The XS coating gives a warm, balanced phosphor bronze strings maximum life, with an uncommonly smooth feel. Made with a NY Steel core, phosphor bronze wrap wire, and Fusion Twist technology, XS coated acoustic guitar strings have incomparable break strength and tuning stability, in addition to their dramatically increased lifespan. 13-56 Medium gauge strings provide maximum volume and projection, with more tension and resistance.