Diplomatic correspondence

Letters are correspondence between heads of state, typically used for the appointment and recall of ambassadors; for the announcement of the death of a sovereign or an accession to the throne; or for expressions of congratulations or condolence.

[1] A letter of credence (lettres de créance) is the instrument by which a head of state appoints ("accredits") ambassadors to foreign countries.

In cases where an envoy is entrusted with unusually extensive tasks that would not be covered by an ordinary permanent legation (such as the negotiation of a special treaty or convention, or representation at a diplomatic congress), an envoy may be given full powers (pleins pouvoirs) "in letters patent signed by the head of the State" designing "either limited or unlimited full powers, according to the requirements of the case.

According to Satow's Diplomatic Practice, the bestowal of full powers traces its history to the Roman plena potestas; its purpose was to be able to dispense, as far as possible, with the long delays needed in earlier times for referring problems back to higher authority.

[6] The collective note has been a rarely used form of diplomatic communication due to the difficulty in obtaining agreements among multiple states to the exact wording of a letter.

Examples include the identic note sent by Thomas Jefferson regarding action against the Barbary Pirates and that from the United States to China and the Soviet Union in 1929.

[9] Similar to a démarche, an aide-mémoire is a proposed agreement or negotiating text circulated informally among multiple states for discussion without committing the originating delegation's country to the contents.

In modern times, English has largely replaced French as a diplomatic lingua franca in correspondence between two states lacking a common tongue.