Should An Nvme M.2 M Key Have 4 Pins Or 5?

So this may be a bit of a dumb question. As I understand it (putting aside B+M for the moment) most if not all NVMe M.2 drives are typically keyed in M, and every M.2 NVMe I've come across has 4 pins on the smaller partition of the contacts. Therefore type M = 4 pins. Everything I can find online however says that M key M.2s should have 5 pins, and B key should have 6. Can somebody explain this to me like I'm a 5 year old please.