D'addario xl prosteels utilize a highly magnetic, corrosion-resistant steel alloy that delivers super-bright tone without shrill overtones. they offer a palette of harmonically rich, brilliantly penetrating highs combined with pronounced, tight-and-tough lows. if you need a little more crunch, bite, and sustain, prosteels are the string for you!From the ManufacturerEPS510's offer a super-bright, crunchy tone and corrosion resistance for longer string life.D'Addario XL ProSteels utilize a highly magnetic, corrosion-resistant steel alloy that delivers super-bright tone without shrill overtones. They offer a palette of harmonically rich, brilliantly penetrating highs combined with pronounced, tight-and-tough lows. If you're looking for more crunch, bite, and sustain, ProSteels are the choice for you.D'Addario, the world's largest manufacturer of musical instrument strings, is known as The Players Choice with a reputation for unsurpassed quality and consistency. D'Addario's innovative packaging reduces waste and provides protection from corrosion. All D'Addario strings are manufactured in the U.S.A. using proprietary, digitally controlled, state-of-the-art machinery for unmatched performance, set after set.