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. 13-56 Medium gauge strings provides maximum volume and projection, with more tension and resistance.