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. This 09-45 Extra Light 12-String set is D'Addario's lightest gauge for 12-string acoustic guitar, providing easy playability while retaining adequate tone and projection for most playing situations.