LOOSE STRING FOR CLASSIC GUITAR Single strings Spanish Guitar Pro ArteD-4 Series Wound Extra Strong Tension. Pro-Arte strings are our worldwide sales leader and the best D Addario strings for Spanish guitar. All Pro-Arte strings are classified by a highly sophisticated computer controlled laser machine that performs diameter / tension measurements and quality control, ensuring great sound and precise highs in every set. Pro-Arte bass strings are wound using silver-plated copper over a multi-filament nylon core for a warm, projected tone.