The Apeks RK3 is a rugged thermoplastic rubber fin featuring an over-sized foot pocket to accommodate drysuit boots and a spring strap for easy donning and doffing. The RK3 is available in four sizes: Small, Medium, Large and Super. The environment that an RK3 is used in will tend to dictate the size of fin that you need, for example do you dive in a warm environment wearing wetsuit boots, or are you walking across pebbled beaches and wearing a drysuit and rigid dry boots? Both forms of footwear may be a size 9, but will take up different amounts of space in the fin foot pocket.