ELECTRIC GUITAR STRING SET D'Addario EPN21 XL Series Pure Nickel Super Jazz Light Electric Guitar String Set (012 - 016 - 023 - 031 - 041 - 051). D'Addario XL Pure Nickel strings go back to the '50s, when nickel was the main alloy in electric guitar strings. In the 1960s, these pure nickel strings would be replaced when guitarists sought a brighter tone and increased the characteristic magnetic properties of the D'Addario XL line of steel strings. Pure nickel delivers a warmer, classic tone that defines many genres including blues, classic rock, rockabilly, and more.