The Martin MA540 SP Phosphor Bronze Authentic Acoustic Guitar Strings are engineered with the performer in mind and they will always stand up to rigorous practice and performance schedules. We start with our highest tensile-strength core wire, then tin-plate it on all six strings for added corrosion resistance; because what you don't see matters! When coupled with our highest quality wrap wire you get consistent true tone that you can count on song after song. Available in 80/20 Bronze and 92/8 Phosphor Bronze. Martin Authentic Acoustic SP® strings offer excellent tuning stability, corrosion resistance and great playability.