Martin MSP4100 Phosphor Bronze Light Acoustic Guitar Strings are made from a 92/8 composition alloy to give that brilliance, clarity, and longevity of sound required by serious guitarists. The phosphor bronze winding alloy is ideally suited for making strings that have deep, rich basses and clear, bright trebles that today’s acoustic guitarists demand. Martin strings are high-quality strings designed for daily use. Martin acoustic guitar strings are wound to precise specifications. Core and wrap wires must meet strict requirements to make the grade.