Royal Highness (abbreviation HRH) is a style (His Royal Highness or Her Royal Highness); plural Royal Highnesses (abbreviation TRH, Their Royal Highnesses). It appears in front of the names of some members of some royal families other than the King or Queen.
The style His/Her Royal Highness ranks below His/Her Imperial Highness (referring to an Imperial House) but above His/Her Grand Ducal Highness, His/Her Highness, His/Her Serene Highness and some other styles (referring to Grand Ducal, Princely or Ducal Houses).
In the British monarchy the style of HRH is associated with the rank of prince or princess (although this has not always applied, the notable exception being Prince Philip, Duke of Edinburgh, who was given the style of HRH in 1947 but was not created a prince until 1957). This is especially important when a prince has another title such as Duke (or a princess the title of Duchess) by which he or she would usually be addressed. For instance HRH The Duke of Connaught was a prince and a member of the royal family while His Grace The Duke of Devonshire is a non-royal duke and not a member of the British Royal Family. Both The Lady Louise Windsor and Viscount Severn, children of the The Earl of Wessex, are legally entitled to the style Royal Highness but it was decided by their parents that they would be styled as the children of an earl and not by their legal style. The Duke of York's daughters Princesses Beatrice and Eugenie enjoy the style Her Royal Highness.
In the United Kingdom, letters patent dated 21 August 1996 states that a style received by a spouse of a member of the Royal Family on their marriage ceases at the point of divorce. For that reason HRH The Princess of Wales, when she and Prince Charles, Prince of Wales divorced, ceased to be HRH, and was styled Diana, Princess of Wales.