From D'Addario:

D'Addario was the first to use Phosphor Bronze in acoustic guitar strings back in 1974. Today, Phosphor Bronze is played throughout the world and considered to be the standard in acoustic strings, thanks to its beautiful sonic balance and tonal warmth. Our pioneering Phosphor Bronze acoustic guitar strings are precision wound with phosphor bronze wrap wire over a high-carbon steel core, providing tonal versatility suitable for all acoustic styles and musical genres. Designed for all makes and models of 12-string acoustic guitars, this 10-47 Light gauge set is D'Addario's most popular 12-string set, offering the ideal balance of volume, projection and comfortable playability.