It's a 2mm fin sock and a fin heel pad in one! You can move them from one pair of fins to another. They are not permanently attached. The sock is attached to the fin by a Velcro pad that wraps around the heel of your fin. Give your feet comfort and protection. Available sizes: (NOTE: THE CURRENT BATCH OF THESE RUN ABOUT 1.5 SHOE SIZES SMALL, SO FOR EXAMPLE, SIZE LARGE SAYS 10-11, BUT FITS 8.5-9.5) (men's shoe size- see our size chart link for women's conversion): Small: 4-7 US shoe size Medium: 8-9 US shoe size Large: 10-11 US shoe size XL: 12-13 US shoe size