Martin MSP7050SW-1PK SP Lifespan Phosphor Bronze Acoustic Guitar Strings, Custom Light, 11-52, 1 Pack Made with bronzed steel core wire and phosphor bronze wrap wire 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! Custom Light: .011, .015, .023, .032, .042, .052