Martin M240 Bluegrass Acoustic Guitar Strings 3 Packs Light - Medium String Gauge: .012ss, .016ss, .025w, .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. Martin Bluegrass sets combine rich, deep bass strings with bright treble strings in specially matched sets to give bluegrass musicians the sound that discerning audiences appreciate. Ball End; Tension: 176.9