Martin M150 Bronze Acoustic Guitar Strings Med 3 Packs Medium String Gauge: .013ss, .017ss, .026w, .035w, .045w, .056w 80/20 Bronze: This alloy is comprised of 80% copper and 20% zinc. It is popular for acoustic strings and provides a very brilliant tone when they are initially put on the guitar and mellow over time. Silvered Steel (ss): High carbon steel with tinplating has very good tensile properties. This core wire is used for both acoustic and electric sets. It is used for the core wire of wrapped strings and the plain round treble strings. Tension: 181.1 Ball Ends