Martin M180 12-String 80/20 Bronze Extra Light Acoustic Guitar Strings are made from a composition alloy to give brilliance, clarity, and longevity of sound. The Bronze winding is ideally suited for making strings that have deep, rich basses and clear, bright trebles. Martin Strings are high-quality strings for daily use. Core and wrap wires must meet strict requirements to make the grade. Martin Strings are wound to precise specifications.