Martin MSP7200SW SP Lifespan Phosphor Bronze Acoustic Guitar Strings, Medium, 13-56 Medium: .013, .017, .026, .035, .045, .056 Made with bronzed steel core wire and phosphor bronze wrap wire All six strings are treated for long life Treated for long life, they repel dirt, grit and other environmental elements that can quickly alter the sound of your strings Designed to last longer and sound better so you can play on!