The XS coated strings were originally developed for acoustic guitars, but now they are also offered as the XS Electric with maximum life, strength, tuning stability and great tone. D'Addario uses their proprietary NY Steel for the core wire and plain string wire for the highest break strength, and hex-core construction for superior dimensional stability, enhanced durability, and precise intonation. They also use their proprietary Fusion Twist technology on the plain steels for significantly enhanced tuning stability.
The XS Electric strings include a reformulated nickel-plated steel wrap wire to provide a bright tone, high output, bite, and sustain.
Long string life comes from a super-thin film coating (1/10 the thickness of a human hair) that protects the wound strings from corrosion.
I tried out a set of XS nickel-plated steel (10-46s) on my Fender Strat of their Super Lights (9-42s) on my Tele and liked them as well-especially for easier string bending!
The XS line, now with both acoustic and electric sets, offers a choice between XT - for enhanced life and a natural feel-or XS-for maximum life and a smoother feel. Either set has great tuning stability and rich D'Addario tone.