D'Addario's XL strings are round wound with nickel-plated steel wrap wire. They produce a bright, true sound. D'Addario EXL120-7 XL's are precision wound with super light gauge nickel-plated steel on a hex-shaped core, resulting in strings with a distinctive bright tone and excellent intonation.About D'Addario StringsThe devil is always in the details, and sweating the details has been a family tradition at D'Addario for nine generations. Just look at something as simple as their plain steel guitar strings. They pay more to have the ball ends precision turned from solid brass stock. The edges have radii that prevent the ball end from cutting into wooden bridges or guitar tops.Each string is twisted to perfection right down to the 3 neatly wrapped "lock twists." And those lock twists help keep the plain steel strings from unraveling or breaking prematurely. Each and every spool of precision drawn high carbon steel wire passes 3 quality control tests before it hits the inventory shelves. Pioneering innovative string design has been a D'Addario family tradition for nine generations. Most guitar players don't realize that the inner core wire in all D'Addario steel strings is made from carefully drawn, hexagonally shaped, high carbon steel wire. The 6 "edges" of the core wire bite into the softer wrap wire, securing it all along its length. The hexagon core is just one of the reasons D'Addario strings are "Better by Design." Hexagonally shaped high carbon steel core wire. Core wire edges dig in to the wrap wire to ensure a tight bond between the core and the wrap wire.