The whole entire sock is elastic with an Velcro strap. It becomes less elastic due to silicone dipping Size, S - M - L - XL An incredibly strong bootie for dryland Width/Circumference: S: 70 mm/150 mm/140 mm M: 80 mm/ 160 mm / 140 mm L: 85-95 mm/170-190 mm/150 mm XL: 95-105 mm/190-210 mm/160 mm