Jack's Guitarcheology is an official D'Addario dealer.

From D'Addario:

Born from our greatest breakthrough in string innovation, XS Phosphor Bronze strings have the longest life of any D’Addario mandolin string. The XS coating gives our 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 mandolin strings have incomparable break strength and tuning stability, in addition to their dramatically increased lifespan. 11-40 Custom Medium gauge strings provide more volume and resonance than Light strings, while still maintaining playability.

• XS Phosphor Bronze strings have the longest life of any D’Addario acoustic string, thanks to the ultra-thin XS film coating.
• High carbon steel wire and Fusion Twist technologies deliver unrivaled tuning stability and break resistance. Strings stay in tune 131% better than traditional strings!
• Less acoustic damping than other coated strings, delivering true and pure iconic D’Addario tone.
• The inner packaging features a resealable zip closure, for ease of opening as well as storage of unused strings.
• String Gauges: .011, .011, .015, .015, .026w, .026w, .040w, .040w