Round Neck T-Shirt The High O2 Dryfit Series is optimized for peak athletic performance : 1) High O2 Breathability: Micro-ventilation weave designed to increase oxygen flow to the skin. 2) Rapid Dry Technology: Lightweight polyester fabric that pulls moisture to the surface for fast evaporation. 3) Ergonomic RN (Round Neck): Flat-seam construction at the collar to prevent chafing and irritation. 4) 4-Way Stretch: Flexible knit fabric that moves with your body, providing a full range of motion.