AIRfeet OUTDOOR O2 utilize our Dynamic Arch Support Technology. Without a doubt a "must have" for those with rugged outdoor lifestyles. It's close cousin the "TACTICAL O2" is supporting 170+ Army & Airforce locations globally. 4 Unisex SKUs. - S (M 3.5-6.5; W 5-8) - M (M 7-9; W 8.5-10.5) - L (M 9.5-12.5; W 11-14) - XL (M 12.5W-15.5; W 14W-17.5) SNIP & UNZIP trimming provides the customer with the most intelligent yet simple method of trimming to their size if needed. Targeting: - Plantar Fasciitis, Comfort, Flat Feet, Neuropathy, Foot & Knee Pain, Back & Hip Pain. Designed For: - Standing, Walking, Working, Hiking, Hunting, Living Benefits: - 80% Dynamic Arch Support, - Feather Light, - Wicking Upper Fabric, - Super Thin, - Reversible, - Machine Washable, - FSA/HSA Eligible, - Compliments Orthotics - 6 Month Warranty Polyester Upper and Lower wicking fabric Patented TPU Air Chamber Design