It varies from state to state. When I had butterfly knives, I lived in florida and I looked up that statute to determine that mine, were LEGAL under florida law. It has been many years ago, but I think Florida allows concealed knives so long as they are not over 4 or 4 1/2 inches long from hilt to blade tip and so long as it is not a switchblade type. That is my recollection, but I was certain of their legality before I started purchasing them.
Each state is different.