From D'Addario:

Rooted in generations of D’Addario string making expertise, Pro-Arte’ Nylon is our archetypal classical set. The bass strings are made with a multifilament nylon core and wound with silver-plated copper, for an exceptionally warm and consistent response. Coupled with clear nylon treble strings, which deliver sweet, mellow highs, Pro-Arte’ Nylon gives you the sound and feel that has come to define classical guitar. Hard tension strings provides rich, bold tone and strong projection.