Round Wound The most popular and universal construction, Round Wound strings deliver the traditional textured feel most players are familiar and comfortable with. Nickel-plated Steel Renowned for their distinctive, bright tone, XL nickel-plated steel is the industry standard for many genres and playing styles. Hex Core D'Addario Nickel Wound strings are wound with nickelplated steel onto a carefully drawn, hexagonally shaped, high carbon steel core. A high carbon steel alloy that is six-sided in cross section. When usedas the core or â€œcenterâ€ of a musical string, the 6 edge points firmly grip every turn of the wrapwire, ensuring a tight wrap and longer-lasting string.