This bundle includes three packs of Martin MA550 Authentic Acoustic SP 92/8 Phosphor Bronze Medium 13-56 strings.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 donâ€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.