Quick Primer on Royal and noble titles
(from Merriam Webster)
By tradition, only those born into the royal family can use “prince” or “princess” before their name and, other than the Prince of Wales—presumably to show his primacy—they are officially known by other titles: Prince William is the Duke of Cambridge, for example, and Prince Harry is named the Duke of Sussex on his wedding day.
Even though many people refer to “Princess Diana” or “Princess Kate,” the title of princess should properly come after their names, since they were not born into the royal family; they are officially referred to as “Diana, Princess of Wales” and, for Kate, “Princess William of Wales."
After royalty comes nobility, and these titles also derive from their French antecedents. The five noble ranks, in descending order, are:
A duke or duchess is addressed as “your grace,” as is an archbishop, except for those royal dukes (members of the Queen’s family), who are referred to as “royal highness.” The distinction of being referred to simply as “your highness” might logically be assumed to be the reigning monarch, but in Britain, the Queen is only addressed as “your majesty.”
A marquess is “a member of the British peerage ranking below a duke and above an earl.” It’s less well-known as a title than duke or earl, probably because there are fewer marquessates than dukedoms or earldoms in Britain. Marquess is an anglicization of the French marquis, pronounced \mahr-kee\ with a silent s, but marquess is pronounced in the English manner as \MAHR-kwus\. The feminine form of the title in French, marquise, is pronounced \mahr-keez\, but the feminine form in English is marchioness, pronounced \MAHR-shuh-nus\. Marchioness is derived from Latin rather than French, probably because the French-derived title marquisess sounded too much like marquess.
Of these titles, only earl is an etymologically English word. Earl is an ancient title that comes from the Old English word for "warrior" or "nobleman." An earl is the English equivalent in rank to a European count; for some reason, there is no feminine form of earl, so the British aristocracy uses the more continental sounding countess.
Viscount is pronounced \VYE-count\, and is, etymologically speaking, a "vice-count" or inferior count.
Sir is used to address a man who has the rank of baronet or knight; the higher nobles are referred to as Lord. Lady is used when referring to women who hold certain titles: marchioness, countess, viscountess, or baroness. It can also be used of the wife of a lower-ranking noble, such as a baron, baronet, or knight.
Lady is also the courtesy title for the daughters of the higher-ranking nobles duke, marquess, or earl. The daughters of viscounts and barons are referred to as "The Honorable" (that is, ahem, “The Honourable”), and daughters of baronets or knights are simply called "Miss."
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