Covert Threads™ Rock Infiltrator Rock is our Hi-Speed Sock for Rugged Terrain. Specifically for Long Hauls & Heavy Loads, holds its shape through high mileage days, and provides energizing support of muscles. High Loft resilient cushion in specific strike zones help eliminate bruising of foot bottom from rugged territory. Silver Lining™ fights bacteria all day, every day. Go covert. * Medium=Men's Shoe 4-8 / Women's 5-10 * Large= Men's Shoe 9-12 / Women's 11+ * XLarge= Men's Shoe 13-15.