These are fin socks for bodyboarding, bodyboarding, diving, and snorkeling. The size marking has faded, but I bought them to fit my 25.5cm (US size 7.5-8) feet. I think they would be around a size M. The thickness is probably 3mm. They have a tether (fin saver) function. They have been used 2 or 3 times and are in good condition. I don't see any defects (holes, fraying, etc.) in particular. Thank you.