D'Addario: EXP Phosphor Bronze 13-56 Medium EXP17
An ultra-fine layer of EXP Coating is bonded to the phosphor bronze wrap wire which is then wound onto the hex-shaped core wire. The result is a built-in barrier against the corrosion and wear that can quickly sap the life out of your strings and your sound. EXP coated phosphor bronze strings have a natural round wound feel and sound fresh and bright with a "new string tone" that lasts three to four times longer than traditional guitar strings.