Plain strings are iron / cobalt alloy, wound strings are wrapped with the iron / cobalt wire. Their Power Slinky gauge 011 014 018 028w 038w 048w. Maker says: "Cobalt provides a stronger magnetic relationship between pickups and strings than any other alloy previously available. Cobalt strings provide an extended dynamic range, incredible harmonic response, increased low end, and crisp, clear highs. Cobalt Slinkys are also soft and silky to the touch, making string bending a breeze."