Martin 13-56 SP Phosphor Medium

The "13-56" refers to the gauge or thickness of the strings, with the first string being 0.013 inches in diameter and the sixth string being 0.056 inches in diameter. "SP" stands for "Stay Play," which is a type of coating on the strings that helps them last longer and resist corrosion. "Phosphor" refers to the material used in the winding of the strings, which is a type of alloy that contains phosphorus and provides a warmer, more balanced tone compared to 80/20 bronze strings. Finally, "Medium" refers to the tension or feel of the strings, with medium tension strings providing a balance between ease of playing and tone.