D'Addario EJ39 Phosphor Bronze 12-String Acoustic Guitar Strings, Medium, 12-52 Phosphor Bronze was introduced to string making by D'Addario in 1974 and has become synonymous with warm, bright, and well balanced acoustic tone. EJ39, D'Addario's medium gauge 12-string acoustic guitar set provides heavier tension resulting in a bolder, more resonant acoustic tone with higher resistance for heavy strumming or aggressive fingerpicking. String Gauges: Plain Steel .012/.12, .016/.016, Phosphor Bronze Wound .025/.010(p), .032/.014(p), .042/.020(p), .052/.030(w)