handed his credentials to U.S. President
George H. W. Bush in 1991. , Princess of Liechtenstein and ambassador extraordinary and plenipotentiary to the Czech Republic, presenting her credentials to
Václav Klaus. The
Congress of Vienna of 1815 formalized the system of
diplomatic rank under
international law, distinguishing between three hierarchical descending categories of diplomatic representatives: full ambassadors (including legates or
nuntii), accredited to heads of state; envoys or ministers, who were also accredited to heads of state; and finally
chargés d’affaires, who were accredited to the
minister of foreign affairs. The
Vienna Convention on Diplomatic Relations of 1961, which took effect in 1964 and is still in force, modified the system. According to it, ambassadors are diplomats of the highest rank, formally representing their head of state, with
plenipotentiary powers (i.e. full authority to represent the government). In modern usage, most ambassadors on foreign postings as
head of mission carry the full title of Ambassador Extraordinary and Plenipotentiary. The distinction between extraordinary and ordinary ambassadors was common when not all ambassadors resided in the country to which they were assigned, often serving only for a specific purpose or mission. The
ambassador extraordinary and plenipotentiary being historically regarded as the personal representative of the sovereign, the custom of dispatching ambassadors to the head of state rather than the government has persisted. For example, ambassadors to and from the
United Kingdom are accredited to or from the Royal
Court of St James's. Ambassadors hold the highest
diplomatic rank and have precedence over ''
chargés d'affaires'', who are accredited by the
foreign minister. Ambassadors also outranked
envoys until the 1960s, when the last
legations were upgraded to embassies. Because members of the
Commonwealth of Nations have or had a common head of state, they do not exchange ambassadors, but instead have
High Commissioners, who represent the government, rather than the head of state. The diplomat representing the
Holy See is titled a
nuncio. In diplomatic usage, both the high commissioner and nuncio are considered equivalent in rank and role to an ambassador; high commissioners, like ambassadors, carry the full title of "High Commissioner Extraordinary and Plenipotentiary", but nuncios do not. Resident Coordinators within the United Nations system are accredited to the Head of State and have the same rank as ambassador. Ambassadors carry formal
letters of credence from their
head of state, addressed to the host country's head of state. Because many Commonwealth countries have the same head of state, the accreditation of a High Commissioner is in the form of a simple and often informal letter of introduction from one
head of government (Prime Minister) to that of another. The difference in accreditation is also reflected in the formal titles of envoys to foreign and Commonwealth states: e.g., British High Commissioners are formally titled "The High Commissioner for
His Majesty's Government in the United Kingdom", whereas British Ambassadors to foreign countries are known as "His
Britannic Majesty's Ambassador". == Ambassador-at-large ==