From D'Addario:
XL Nickel Wound electric guitar strings are the standard against which all other strings are measured. Our best-selling 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.
Popular with studio musicians for doubling 6-string guitar parts to achieve a 12-string effect, this 10-26 High Strung/Nashville Tuning gauge utilizes the higher 'octave' strings of a 12-string set to achieve a shimmering, "high-strung" tone.