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. Our impeccable standards for acoustic guitar tone are embedded in our string design and precision manufacturing process. That's why we make our own. .010, .014, .023, .030, .039, .047 gauges with 135.8 tension.