Martin MSP4050 SP Phosphor Bronze Custom Light Acoustic Guitar Strings are made from 92/8 composition to give brilliance, clarity, and longevity of sound. The phosphor bronze winding alloy is ideally suited for making strings that have deep, rich basses and clear, bright trebles. Martin strings are high-quality strings designed for daily use. Core and wrap wires must meet strict requirements to make the grade. Martin acoustic guitar strings are wound to precise specifications.