Martin Authentic Acoustic Studio Performance (SP) strings are engineered with the performer in mind, and they will always stand up to rigorous practice and performance schedules. The core is Martin's highest tensile-strength which is then tin-plated for corrosion resistance (including the non-wounds). The wound string cores are then combined with a layer of silk and then wrapped with Martin's highest quality Silver-plated Copper wire which consistently delivers a bright, but soft, Folk-y tone.