Through advanced EXP Coated Technology, D'Addario has perfected the art and science of the coated guitar string making extended-play strings that sound identical to the renowned D'Addario XL series. A micro-fine layer of concentric EXP coating is applied to the wrap wire prior to winding, resulting in replacement strings that deliver longer life, completely natural tone, and the feel of uncoated strings. Plus, the whole set lasts 4 times longer thanks to D'Addario's exclusive compound-plated, plain steel strings, which offer maximum corrosion resistance. You won't believe they're coated. Includes free extra first string.