From Martin:


Martin Retro® strings are the real deal. Not an imitation. With Martin Retro® strings, you get one tough set that offers true, consistent tone and sounds like nothing else on the market. Martin’s solid nickel/copper alloy blend provides a reduced pick attack that allows the guitar’s tonewoods to be heard, not overshadowed. Martin’s proprietary Monel wrap wire is naturally corrosion resistant, so you can play them for a long time with nothing to get in the way of your music. Discover your guitar’s true voice.
Retro® strings offer enhanced longevity with warm natural tone, and they sound great plugged in.