Vienna Convention on Diplomatic Relations
The Vienna Convention on Diplomatic Relations, which codified longstanding custom, operates under the presumption that the head of a diplomatic mission (i.e. ambassador or nuncio) of the sending state is accredited to the head of state of the receiving state. The head of state accredits (i.e. formally validates) his or her country's ambassadors (or rarer equivalent diplomatic mission chiefs, such as high commissioner or papal nuncio) through sending formal a Letter of Credence (and a Letter of Recall at the end of a tenure) to other heads of state and, conversely, receives the letters of their foreign counterparts. Without that accreditation, the chief of the diplomatic mission cannot take up their role and receive the highest diplomatic status. The role of a head of state in this regard, is codified in the Vienna Convention on Diplomatic Relations from 1961, which (as of 2017) 191 sovereign states has ratified.