There are two types of fins, open heel (pocket) fins and closed heel fins. Open heel fins are the more popular fin design of the two. These fins slide right over your foot and have an adjustable fin strap that tightly secures your foot in the pocket. With the open heel fins you will have to wear lycra or the 3mm socks which are made to fit snugly inside the foot pocket. Wearing the socks with your open heel fins is helpful in keeping your feet warm while in the water and also keeps your heels from rubbing against the heel strap, which can lead to blistering and ruin your snorkeling experience. To check your open heel fins for a proper fit, try on the fin only after you have a sock on. The fin should fit over your ankle without your toes touching the edge of the foot pocket. If your foot does not slide in all the way or feels to tight, it is too small and will be uncomfortable and strain your leg muscles, which does not equal an enjoyable snorkeling trip.