From D'Addario:

Since D'Addario introduced Phosphor Bronze acoustic guitar strings in 1974, they've been synonymous with warm, well-balanced tone. XT Phosphor Bronze offers that and more, boasting enhanced break resistance, better pitch stability, and longer life than our standard phosphor bronze strings. The XT coating delivers advanced corrosion resistance, while still retaining the warm tone and natural feel of the uncoated string. Beneath the coating, these XT acoustic strings are constructed with a NY Steel core, phosphor bronze wrap wire, and Fusion Twist technology, affording tonal versatility with added resilience, excellent for a wide range of acoustic styles. This 12-53 Light gauge set is our most popular acoustic gauge, providing a nice balance of playability and tone.