Diplomatic law is that area of international law that governs permanent and temporary diplomatic missions. A fundamental concept of diplomatic law is that of diplomatic immunity, which derives from state immunity. It is also an accepted principle of customary international law and is recognised between countries as a matter of practicality. Diplomatic law is often strictly adhered to by states because it works on reciprocity. For example, if a country expels diplomats from another country, then its diplomats would most likely be expelled from the other country.