Ernie Ball Nickel Plain Single Guitar String .013 Gauge 6-Pack Nickel Plain Single Guitar String 013 Gauge 6-Pack Made of specially tempered tin plated high carbon steel Used on electric and acoustic guitars, banjos and mandolins but, not classical guitars (Custom Gauge Single Strings) Made of specially tempered tin plated high carbon steel Used on electric and acoustic guitars, banjos and mandolins but, not classical guitars (Custom Gauge Single Strings)