From Martin: These 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. � Features & details This bundle includes three packs of Martin MA140 Authentic Acoustic SP 80/20 Bronze Light 12-54 strings