A thin flat blade is what justifies a clam knife. It is well-suited for a job wherein you have to hold the clam firmly in your palm and insert the knife. This action obviously requires a flat surface, then force your clam knife between the shells and pry them open. On the contrary, oyster knives and short as you need to twist the knife between the shells with a fair amount of force until the shells pop open. Both the knives cannot have a pointed blade as it can be a hindrance in action or also damage the shells, which you don’t want to happen.