The style His Majesty or Her Majesty (HM) is enjoyed by a king, a queen regnant, a queen consort, and a queen dowager. Use of the style His Royal Highness or Her Royal Highness (HRH) and the titular dignity of Prince or Princess are governed by Letters Patent issued by George V on 30 November 1917 and published in the London Gazette on 11 December 1917. These Letters Patent state that henceforth only the children of the Sovereign, the children of the sons of the Sovereign, and the eldest son of the eldest son of the Prince of Wales would "have and at all times hold and enjoy the style, title or attribute of Royal Highness with their titular dignity of Prince or Princess prefixed to their respective Christian names or with their other titles of honour." They further state, "the grandchildren of the sons of any such Sovereign in the direct male line (save only the eldest living son of the eldest son of the Prince of Wales) shall have the style and title enjoyed by the children of Dukes."
Under these conventions, HM The Queen's children, the children of HRH The Prince of Wales, HRH The Duke of York and HRH The Earl of Wessex would all be titled Princes or Princesses and styled Royal Highness, as would be the eldest son of HRH The Duke of Cambridge. However, upon HRH The Earl of Wessex's marriage in 1999, it was announced that his children would always be styled as an earl's children. The Duke of Gloucester, The Duke of Kent, Princess Alexandra, The Hon Lady Ogilvy and Prince Michael of Kent enjoy the titular dignity of Prince or Princess and the style Royal Highness as male-line grandchildren of HM King George V. However, none of their children (being merely great-grandchildren of a monarch) have royal titles. For example, the children of Prince Michael of Kent are known as Lord Frederick Windsor and Lady Gabriella Windsor, the courtesy titles as children of a duke. They are not entitled to any royal title. The children of HRH The Princess Royal, HRH Princess Alexandra, and the late Princess Margaret, Countess of Snowdon, are not entitled to any royal title since princesses do not transmit their titles to their children. An exception to this rule was when HM King George VI issued Letters Patent such that his heiress presumptive, HRH The Princess Elizabeth, Duchess of Edinburgh, could transmit her title to her children. HRH Princess Margaret, Countess of Snowdon's son enjoys the courtesy title Viscount Linley as the son and heir of The Earl of Snowdon, while her daughter enjoys the courtesy title Lady. The children of HRH The Princess Royal and of HRH Princess Alexandra have no titles, because Mark Phillips and Sir Angus Ogilvy did not accept hereditary peerages upon marriage.
Women marrying sons and male-line grandsons of a Sovereign are normally styled Her Royal Highness followed by the feminised version of her husband's highest title. The wives of royal peers are known as "HRH The Duchess of ..." or " HRH The Countess of ..." Thus, the wives of HRH The Duke of Cambridge, HRH The Earl of Wessex, HRH The Duke of Gloucester and HRH The Duke of Kent are "HRH The Duchess of Cambridge," "HRH The Countess of Wessex," "HRH The Duchess of Gloucester," and "HRH The Duchess of Kent," respectively. Before their divorces, Diana, Princess of Wales and Sarah, Duchess of York enjoyed the respective titles and styles of "HRH The Princess of Wales," and "HRH The Duchess of York". However, when a woman marries a prince who does not hold a peerage, she is known as HRH Princess [Her husband's Christian name], followed by whatever territorial or titular designation. For example, the former Baroness Marie-Christine von Reibnitz enjoys the title and style of "HRH Princess Michael of Kent," and not "HRH Princess Marie-Christine of Kent." Similarly, the former Birgitte Eva van Deurs was titled "HRH Princess Richard of Gloucester" from her wedding until her husband succeeded to his father's dukedom in 1974. The widows of princes remain HRH. However, under HM The Queen's 21 August 1996 Letters Patent, a divorced wife of a Prince of The United Kingdom of Great Britain and Northern Ireland "shall not be entitled to hold and enjoy the style, title or attribute of Royal Highness."
There has been one exception to the convention that wives of princes take their husband's rank. In Letters Patent dated 28 May 1937, King George VI specifically denied the style HRH to the wife of HRH The Duke of Windsor, the former King Edward VIII. Therefore, the former Wallis Warfield Simpson was known as "Her Grace The Duchess of Windsor," not "HRH The Duchess of Windsor."
Another exception was made in the case of the last blue-blood to marry into the British Royal Family: Princess Marina who, being a Princess of Greece and Denmark by birth, reverted to her own princely style ("HRH Princess Marina, Duchess of Kent") after being widowed, rather than a dowager's style "HRH The Dowager Duchess of Kent". Her sister-in-law (born The Lady Alice Montagu Douglas Scott) was given special dispensation by HM The Queen to use a similar style when she was widowed (HRH Princess Alice, Duchess of Gloucester) despite not being herself a princess, rather than the more usual dowager's style, "HRH The Dowager Duchess of Gloucester".
Following on a reluctance by the public to universally support the second wife of HRH The Prince of Wales, it was announced by Clarence House that, should the Prince become King, his wife HRH The Duchess of Cornwall will not be known as HM The Queen but will take the lesser title of HRH The Princess Consort. Out of respect for the late Diana, Princess of Wales, it was also announced that HRH would not be known as The Princess of Wales.
The daughters and male-line granddaughters of the Sovereign do not lose their royal titles upon marriage. Men who marry the daughters and the male-line granddaughters of the Sovereign, however, do not acquire their wives' royal rank and the style HRH.
HM the QueenEdit
Held by Queen Elizabeth II. Since Elizabeth is a girl the H in HM would stand for: Her Majesty and not His Majesty.
HRH The Duke of EdinburghEdit
Held by Prince Philip. After the marriage of him and Elizabeth he was given this title. He would be the duke of Scotland's Capital City and maybe part of Scotland?
HRH The Prince of WalesEdit
Held by Prince Charles. In 1969, Charles was granted The Prince of Wales and he has held it ever since then.
HRH The Duke of Cambridge Edit
Held by Prince William. In 2011, William was granted The Dukedom of Cambridge and he has held it ever since then. Upon his fathers succession to the throne, it is likely King Charles will grant Prince William the Prince of Wales as eldest child of the Monarch
HRH The Prince George of Cambridge Edit
Prince George is the eldest son of the second-in-line to the throne. The Prince was born in 2013. Upon his marriage it is likely he will be granted a Dukedom.
HRH The Princess Charlotte of Cambridge Edit
Princess Charlotte is the first daughter of the The Duke and Duchess, and following changes to the Royal Succession Act, is and remains fourth-in-line to the throne. Upon her fathers succession to the Crown, it is likely King William will grant his daughter The Princess Royal.