Article 12 Vienna Convention on Consular Relations
The exequatur
1.The head of a consular post is admitted to the exercise of his functions by an authorization from the receiving State termed an exequatur, whatever the form of this authorization.
2.A State which refused to grant an exequatur is not obliged to give to the sending State reasons for such refusal.
3.Subject to the provisions of articles 13 and 15, the head of a consular post shall not enter upon his duties until he has received an exequatur.
See also: Article 11 Vienna Convention on Consular Relations