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. Their 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 12-56 Light Top/Medium Bottom Bluegrass set has powerful low-end, while maintaining flexibility for soloing. Phosphor Bronze was introduced to string making by D'Addario in 1974 and has become synonymous with warm, bright, and well balanced acoustic tone. D'Addario Phosphor Bronze strings are precision wound with corrosion resistant phosphor bronze onto a carefully drawn, hexagonally shaped, high carbon steel core. The result is long lasting, bright sounding tone with excellent intonation.