The RUNNING XTRA sock is a thicker Double Layer sock for people who like more cushion. The inner layer is a smooth, flat knit Dri-WRIGHT^TM against the foot. The outer layer is also constructed of heavier Dri-WRIGHT^TM and has cushioning along the foot for added protection. The construction of two layers of Dri-WRIGHT^TM still allows slight movement to absorb the friction. Feet stay dry as the CoolMax wicks the moisture away from the foot and into the outer layer for evaporation.