Trail dirt inside a sock is a bad thing; it can easily cause blisters or rub the skin raw. To help prevent dirt from getting inside Trail socks, we did not include vents. The Trail Running sock is a High Density protective padded sock. It was designed for use in cool to warm conditions, keeping feet dry and comfortable. Sizing represented as follows: Small = W5-7/M3.5-5.5; Medium = W7.5-9.5/M6-8; Large = W10-12/M8.5-10.5; X-Large = M11-13.