EJ15 is an extra light gauge Phosphor Bronze string providing lower tension ideal for vintage instruments and playability. Since D'Addario introduced Phosphor Bronze guitar strings in 1974, they have been synonymous with warm and well-balanced acoustic tone.Phosphor Bronze has become synonymous with warm, 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.