Retros are the original Nickel acoustic stringthe real dealall the way from the 1930s. Retro strings offer a uniquely mellow, yet crisp, sound that allows your guitars natural tone woods to be heard, not overshadowed. They are crafted with Monel, a solid Nickel/Copper wrap wire - not nickel-plated - meaning there is no chance for defects in the plating to allow for corrosion. Instead, Martins proprietary wrap wire is naturally corrosion resistant and it is incredibly strong. We also employ a sophisticated winding process in the making of our Retro String that controls the coupling between the core and wrap wire to maximize intonation. Martin Retros are the real deal. Not an imitation. With Martin Retros, you get one tough set of strings that will last a long time and give you true, consistent tone that sounds like nothing else on the market. Retro strings are made with tin-plated steel core wire and solid Monel wrap wirea proprietary blend of nickel and copper - that is nat