Designed to cope with demanding practices and schedules. Martin Authentic Acoustic SP Phosphor Bronze strings feature their highest tensile-strength core wire and are tin-plated for enhanced corrosion resistance. These Acoustic SP strings provide players with good tuning stability as well as a true and consistent tone throughout gigs. Manufactured with copper tin and phosphorous Phosphor Bronze strings yield a dark and slightly warmer tone that 80/20 strings. These Acoustic SP strings include gauge 12.5 – 16.5 – 25.5 – 33.5 – 43.5 – 55. Features Set of 6 acoustic guitar strings Light medium gauge 12 1/2 – 55 Metal wrap wire composition of copper tin and phosphorous Tin plating offers added corrosion resistance Specifications Gauges: 12.5 – 16.5 – 25.5 – 33.5 – 43.5 – 55 Tension: 179.3