The HEAD Volo One Snorkeling Fins are Head's version of the famous Mares Volo Power Fin featuring the patented OPB hinge technology for easy and comfortable kick as well as Channel-Thrust for excellent performance with minimum effort. Soft, elastic heel straps attach with quick release buckle system. This allows for one time adjustment and easy on/off. Small/Medium fits shoe size US 3 1/2 - 6 Medium/Large fits shoe size US 6 1/2 - 9 Large/X-Large fits shoe size US 9 1/2 - 12