Martin Authentic Acoustic Lifespan 2.0 Phosphor Bronze Guitar Strings -.013-.056 Medium Value PackNext generation Martin Authentic Acoustic Lifespan 2. 0 strings are engineered using a new, patented anti corrosion treatment only available from Martin. It protects the core and wrap wire to prevent corrosion without compromising tone or feel. Our impeccable standards for acoustic guitar tone are embedded in our string design and precision manufacturing process. Thats why we make our own. Gauges: . 013, . 017, . 026, . 035, . 045, . 056