Phosphoer Bronze acoustic guitar strings in Medium Gauge, .013-.056 EJ17s offer heavier tension resulting in a bolder, more resonant acoustic tone with higher resistance for heavy strumming and flat-picking. 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.