From D'Addario:

XL Nickel Wound electric guitar strings are the standard against which all other strings are measured. Our best-belling electric set, XL Nickel has been revered by players everywhere since 1974. Made with a high carbon steel core and nickel-plated steel wrap wire, XL Nickel strings have a bright, versatile tone, ideal for a wide variety of musical styles. This 09-42 Super Light Double Ball End set is constructed with ball ends on both ends of the strings, for use on double ball-end guitars, and is precisely engineered to Steinberger specifications.