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Highness is a formal style used to address (in second person) or refer to (in third person) certain members of a reigning or formerly reigning dynasty. It is typically used with a possessive adjective: "His Highness", "Their Highnesses", etc. Although often combined with other adjectives of honour indicating rank, such as "Imperial", "Royal" or "Serene", it may be used alone.
Highness is literally and, historically, the quality of being lofty or above, used as a term to evoke dignity or honour, and to acknowledge the exalted or official high rank of the person so described.
Abstract styles arose in profusion in the Roman Empire, especially in the Byzantine continuation. Styles were attached to various offices at court or in the state. In the early Middle Ages such styles, couched in the second or third person, were uncertain and much more arbitrary, and were more subject to the fancies of secretaries than in later times. 
In English usage, the terms Highness, Grace and Majesty, were all used as honorific styles of kings, queens and princes of the blood until the time of James I of England. Thus in documents relating to the reign of Henry VIII of England, all three styles are used indiscriminately; an example is the king's judgment against Dr. Edward Crome (d. f562), quoted, from the Lord Chamberlains' books, ser. I, p. 791, in Trans. Roy. Hist. Soc. N.S. lOX. 299, where article 15 begins with Also the Kinges Highness hath ordered, 16 with Kinges Majestie, and 17 with Kinges Grace. In the Dedication of the Authorized Version of the Bible of 1611, James I is still styled Majesty and Highness; thus, in the first paragraph: "the appearance of Your Majesty, as of the Sun in his strength, instantly dispelled those supposed and surmised mists ... especially when we beheld the government established in Your Highness and Your hopeful Seed, by an undoubted title". It was, however, in James I's reign that Majesty became the official style. It may be noted that Oliver Cromwell and his wife were also styled "Highness" upon his elevation to Lord Protector of the Commonwealth; presidents and other republican heads of state are normally styled "Excellency".
At the conclusion of the Congress of Vienna in 1815, His/Her Highness (abbreviated HH), became prevalent for reigning dukes and members of their dynasties in Germany (e.g. Anhalt, Brunswick, Nassau, the three Ernestine duchies of Saxe-Coburg and Gotha, Saxe-Meiningen, and Saxe-Altenburg, as well as Schleswig-Holstein); for cadets of some German grand ducal houses (e.g., Mecklenburg-Schwerin, Mecklenburg-Strelitz, Oldenburg,Saxe-Weimar-Eisenach); and cadet members of some imperial or royal families (e.g., Russia, Albania, Denmark, the Netherlands, Norway, Prussia, Yugoslavia). That custom remains official in the Danish, Dutch and Norwegian dynasties. The Almanach de Gotha and Burke's Peerage continued to ascribe Highness to members of deposed dynasties of ducal rank.
Among the nobility, the Almanach de Gotha notes that Highness was accorded to the heads of the families of House of Murat (a royal dynasty during the Napoleonic era), Hohenberg and all members of the House of Ligne.
Example of official holders of the style Highness:
Usually members of an imperial or royal dynasty are addressed as Imperial Highness or Royal Highness (French Altesse Imperiale, Altesse Royale; German Kaiserliche Hoheit, Königliche Hoheit; Spanish Alteza Imperial, Alteza Real, etc.) respectively.
While "Highness" (Hoheit) was used for rulers of German duchies, the sovereign Dukes of Modena and of Parma were heads of cadet branches of ruling dynasties of higher rank. They and their cadets therefore used the imperial or royal styles borne by members of those houses, respectively the royal House of Bourbon and the imperial House of Habsburg-Lorraine.
In modern times Serene Highness (Altesse Sérénissime) is used as the equivalent of the German Durchlaucht. In the 17th century it became the general style borne by the heads of the reigning princely states of the Holy Roman Empire (reichsständische Fürsten), as "Illustrious Highness (Erlaucht) became customary for those of the comital houses (reichsständische Grafen, i.e. Counts of the Empire). In 1825 the Imperial German Diet agreed to grant the style Durchlaucht to the heads of all mediatized princely houses domiciled in Germany elevated to the rank of Fürst are also styled Durchlaucht. In 1829 the style of Erlaucht, which had formerly been borne by the reigning Counts of the empire, was similarly granted to the mediatized countly families (Almanach de Gotha, 1909, 107).
Highness was the style accorded princes of the British Royal Family who were the male-line great-grandchildren of a British sovereign (and the wives/widows of great-grandsons), except the eldest son of the Prince of Wales. In 1917 George V revoked authorization for use of that style.
The children and grandchildren in the male-line of a British sovereign were and are addressed as Royal Highness (His or Her Royal Highness, abbreviated HRH), as are the children of the eldest son of the Prince of Wales (decree of 31 May 1898).Notice from the London Gazette: The sovereign has the right to grant or revoke use of the style of Highness, as with other styles.
In most of Africa, many styles are used by traditional royalty. Generally the vast majority of the members of these royal families use the titles Prince and Princess, while the higher ranked amongst them also use either Highness or Royal Highness to describe secondary appellations in their native languages that they hold in their realms, appellations that are intended to highlight their relative proximity to their thrones, either literally in the sense of the extant kingships of the continent or symbolically in the sense of its varied chieftaincies of the names, and which therefore serve a function similar to the said styles of Highness and Royal Highness. For example, the Yoruba people of West Africa usually make use of the word Kabiyesi when speaking either to or about their sovereigns and other royals. As such, it is variously translated as Majesty, Royal Highness or Highness depending on the actual rank of the person in question, though a literal translation of the word would read more like this: He (or She) whose words are beyond questioning, Great Lawgiver of the Nation.
Regardless of the official traditions in the various colonial empires, the style is evidently used to render, often merely informally, various somewhat analogous titles in non-western cultures, regardless whether there is an actual linguistic and/or historical link.
In Samoa, the heads of the four paramount chiefly dynasties carry the title of His/Her Highness, and it has been the official style of the O le Ao o le Malo (Samoan head of state) since the country's independence. Furthermore, in North America, some chiefs of certain tribes or nations use the attribute of Highness.
The Aga Khan was granted the style of His Highness by Elizabeth II, Queen of the United Kingdom in 1957 upon the death of his grandfather Aga Khan III. This has been a traditional gesture by British sovereigns since the first Aga Khan allied himself with Britain against Afghanistan.
While the actual precedence depends on the rank itself, and sometimes more specifically on the monarchy, rather than on the style of address, the holders tend to end up roughly in the following order of precedence: