Martin Acoustic MA550 SP Phosphor Bronze 92/8 Medium 13-56 Guitar Strings, 3 PacksValue 3 pack Martin Authentic Acoustic Superior Performance SP strings feature higher tensile strength and tin plated core wire on all six strings that enable a fuller and more balanced tone, quicker response and increased tuning stability. Gauge: Medium E: . 013 B: . 017 G: . 026 D: . 035 A: . 045 E: . 056 Tension: 190. 9