Martin SP strings are designed for maximum tone and longevity. Ultra-high tensile strength steel cores resist breaking. Acoustic bronze electro-finish provides greater string life. Martin SP strings are ideal live, in the studio, or whenever sustained playability is desired.Super string, warm sound,lasting qualities. MSP4000 - 10 14 23w 30 39 47. MSP4050 - 11, 15, 23, 32, 42, 52 MSP4100 - 12, 16, 25, 32, 42, 54. MSP4200 - 13, 17, 26, 35, 45, 56. MSP4150 - 12.5, 16.5, 25.5, 33.5, 43.5, 55. MSP3250 013 .017 .026 .036 .046 .056