EJ47 Pro-Arte 80/20 Bronze Normal Tension Pro-Arte strings are our the world's best selling and D'Addario's premium classical guitar strings. All Pro-Arte treble strings are sorted by a sophisticated computer-controlled laser machine which performs diameter/tension measurements and quality checks, insuring true sounding, precise trebles in every set. J47 basses are wound using 80/20 Bronze on a multi-filament nylon core for slightly brighter tone.