From D'Addario:

XT are our longest lasting classical strings, combining centuries of D’Addario expertise with modern coating technology. XT Classical features our composite basses, constructed with a long-lasting multifilament core, wrapped with silver-plated copper, and finished with our proprietary XT coating, for bold, projecting tone and extended life span. They’re complimented with our precision Pro-Arte’ nylon treble strings which provide the warm, mellow charm traditionalists love. This Normal Tension set is our best-selling tension, preferred for its balance of rich tone, comfortable feel, and dynamic projection.