EJ44, extra hard tension, is optimal for maximum string resistance and projection without compromising tone. 

Pro-Arte, the world's most popular classical strings are D'Addario's premium classical guitar string sets. All Pro-Arte treble strings are sorted by a sophisticated computer-controlled laser machine which performs diameter and tension measurements, ensuring true-sounding, precise trebles in every set. Pro-Arte basses are wound using silver-plated copper on a multi-filament nylon core for consistent and warm, yet projecting tone.