A special royal baby post on British titles!
Firstly, the new baby’s title is His Highness Prince [NAME] of Cambridge. William and Kate’s official titles are Their Royal Highnesses the Duke and Duchess of Cambridge and are properly referred to as such (or as just the Duke and Duchess of Cambridge). This title was given to them upon their marriage, previously William was HRH Prince William of Wales. As William was born a prince, he remains a prince and she is a princess, but the Queen has chosen to style them the Duke and Duchess of Cambridge, and what she says goes. Becoming a Duke made William a Peer of the Realm, which is better than being just a plain prince (and a commoner). If they had been styled as Prince and Princess, Kate would have been referred to as HRH Princess William of Wales because she was not already a princess in her own right. Prince/Princess is their rank, Duke/Duchess is their title. “Princess Diana” was made up by the press and was never her official title. Princess FirstName is only used in the UK when they are a princess by birth.
The ranking of the British nobility:
In modern times, there isn’t that much meaning behind the titles except in rank. Dukes were first created (in England- the concept is older) by Edward III in the 1300s for his close family members and for a long time Dukes were only members of the royal family. Marquesses held pieces of land on the borders (marches) and because of their defensive position they were ranked higher than earls who held counties (earls are equal to counts in other countries, but the British use the Anglo-Saxon derivative of the Scandinavian word jarl) which were interior pieces of land. The title viscount doesn’t seem to have as much history or meaning as the others and is even now mostly only used as a courtesy title. Barons were originally the men who managed the land for a greater lord. Titles were often awarded to people for service to the Crown, so the greater the service, the greater the title.
Within each rank, age of the title indicates seniority. Life peers are titles given to people for the duration of their own life but which are not passed down to their heirs.
The word peer refers only to those who hold one of these titles fully (or their spouses) and traditionally would be eligible for the House of Lords. Everyone who is not a peer is a commoner (and that includes people like Prince Harry as prince and princess are courtesy titles for the children and grandchildren of the soverign). Children of peers may hold courtesy titles (we will get to those in a minute) but they are not accorded the full honors of that title and they are still commoners. So yes, even though they were rich, and Diana was aristocratic, before their marriages both Kate Middleton AND Princess Diana were commoners.
Children of peers are commoners but they get to use courtesy titles to show their relationship to a peer. Peers often have multiple titles, so they give their eldest sons one of the lesser titles to use as a courtesy, as they will one day inherit the greater title. So the oldest son of a duke might be referred to as the Earl of ____. (This only goes for the eldest son of dukes, marquesses, and earls). Though the son may be styled a Marquess or Earl, they do not hold the full courtesy of that title. For example, a Marquess is properly known as The Most Honourable [first name] Marquess of _______, but a courtesy marquess is not The Most Honourable, they are just the Marquess of ______ (and the “the” is dropped for correspondence)
Younger sons of dukes and marquesses are styled Lord [first name][surname]. Younger sons of earls and all sons of viscounts and barons are styled The Honourable (often shortened to The Hon) [first name][last name]. This is only used descriptively and in addresses, Honourables should be called Mr. ________.
Daughters of dukes, marquesses, and earls are styled Lady [first name][last name].
Sons and daughters of viscounts and barons also use the courtesy title The Honourable in the same way as noted above.
The widowed wife of a duke, marquess, earl, or viscount is the Dowager of that title. For example: widows of dukes are referred to as the Dowager Duchess of ______ or [first name], Duchess of ______. If there is already a Dowager Duchess when the duchess in question is widowed, she is always referred to as [first name], Duchess of ______. If a duchess’s son is unmarried when she becomes widowed, she remains the Duchess of ______ until he marries. (This applies to widows of marquesses and earls also, with Marchioness, Countess, or Viscountess filling in for Duchess.)
Widows of barons are known as Dowager Lady _______ or [first name], Lady _______.
Princes of the Royal Blood are usually created dukes when they marry, as Prince William became the Duke of Cambridge when he married Kate Middleton. There are also non-royal dukes who can trace their lines back to someone who was created a duke by a monarch. All the children and some of the grandchildren of the monarch are addressed as His/Her Royal Highness followed by their other title (the Duke/Duchess, Earl/Countess, Prince/Princess, etc).
In conversation/print Dukes/Duchesses are referred to as The Duke or Duchess of ________ or His/Her Grace. They are addressed directly as Duke or Duchess or Your Grace.
Marquess/Marchioness, Earl/Countess, Viscount/Viscountess, Baron/Baroness
In conversation/print and when addressed directly, these ranks are called Lord or Lady______ (where the blank is their holding, not their first name).
Many earldoms/barons can be inherited by women, so these women are properly called the Countess of/Baroness ______, but her husband gains no title or style from being married to a Countess/Baroness.
A baroness in her own right has the choice of being called Baroness_______ or Lady ________ (where the blank is their last name). Most choose to go by Lady as Margaret Thatcher, a Life Baroness, went by Lady Thatcher.
The Princess Royal refers to the eldest daughter of a monarch. Though as she retains the title for life and there can only be one at a time, if a monarch has a daughter and there is already a Princess Royal, she won’t be called that. Queen Elizabeth’s daughter Anne is the current Princess Royal.
His/Her Royal Highness (HRH) is a style given to members of the royal family.
I have been asked before why Prince Philip isn’t King Philip. The reason is that a King outranks a Queen and the ruler must be the highest ranked person, so when a woman is Regnant, her husband is Prince Consort instead of King. When there is a King, his wife is the Queen Consort as opposed to the Queen Regnant when a woman rules, though generally Queen Consort is just shortened to Queen.
This is an extremely simplified (but hardly simple!) explanation of a very complicated topic. For everything you could possibly want to know about British titles up to how to address the Queen, see http://www.debretts.com. The British Monarchy also has an excellent site specifically about the Royal Family http://www.royal.gov.uk/. And please see this excellent post by a royalty scholar http://royalmusingsblogspotcom.blogspot.com/2011/05/primer-catherine-is-princess.html.