From D'Addario: D'Addario XS Acoustic Phosphor Bronze strings combine our most popular acoustic alloy with our most advanced string coating technology, resulting in clear tone, smooth feel, and superior tuning stability – all with the longest possible string life. With its ultra-thin film coating on every wound string and a unique polymer treatment on the plain steels, XS offers our highest level of protection for maximum life. XS also features NY Steel high carbon core and plain string wire, and our exclusive Fusion Twist, for incomparable break strength and tuning stability. XS is the most innovative coated string we’ve ever created. Never Fade Away!