Martin MSP4000 Phosphor Bronze Extra Light Acoustic Guitar Strings are made with a unique 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 intended for daily use. Core and wrap wires must meet strict requirements to make the grade. Martin acoustic guitar strings are wound to precise specifications.