Prince William got a bevy of titles as a gift from Queen Elizabeth II for his marriage to Catherine (or Kate) Middleton. Real titles come with quite a baggage of history, some quite recent, some more ancient. So William was saddled with the Duchy of Cambridge in England, the Earldom of Strathearn in Scotland, and the Barony of Carrickfergus in Northern Ireland.
The Duchy of Cambridge is neither new nor very inventive. James II styled his first son Charles (born 1660, died 1661) as Duke of Cambridge; he tried again with his second son Charles, whom he created Duke of Cambridge in 1664 (born 1663, died 1667). Subsequently, he created his fourth son Edgar Duke of Cambridge in 1667 (born 1667, died 1671), and styled his fifth son Charles as such as well (born and died 1677).
The difference between created and styled, by the way, lies in the fact that in a created title the legal paperwork has been done while in styled it hasn't. With the paperwork is done, a Duchy becomes a tangible reality. If that Duchy lapses for lack of an heir it becomes available again to the sovereign to bestow on someone else. If this is done, it is named as Duchy second creation.
The third creation of the Duchy of Cambridge was done in 1706 for George, Hereditary Prince Elector of Hanover. His grandmother Sophia was named as heir to the British throne with the Act of Settlement in 1701, and his father George pushed through the naturalization of the Hanoverian family as British citizens which he perceived (rightly) as a crucial step in securing the succession. As part of the organisation of the succession over the heads of Catholic relatives, the Duchy of Cambridge was one of many honors conferred into the Hanoverian line of future kings. With the accession of George as King George II, the duchy merged with the crown.
The Duchy of Cambridge found its fourth creation in 1801, when the seventh son of King George III, Prince Adolphus, was created Duke of Cambridge; his son Prince George would follow him as second Duke while his daughter Princess Mary Adelaide got married to Prince Francis of Teck (later created Duke of Teck by the King of Wurttemberg). Prince George’s most memorable quote was ‘arranged marriages are doomed to failure’. Accordingly, he married a commoner without consent from the monarch and their children were excluded from succession to the title.
When King George V renounced all German titles for the Royal Family in 1917, he also renounced the titles of his brother-in-law Adolphus second Duke of Teck. Consequently, he created him Marquess of Cambridge. His son the second Marquess died without male issue in 1981.
The title Earl of Strathearn is an ancient one; the Mormaer of Strathearn was first mentioned in a document of 1115; the rulers of Strathearn were subsequently named as Earls of Strathearn. The title lapsed for a short time after Maol Íosa V became Earl of Orkney in 1334. Maurice of Moravia held the title and the lands from 1343 to 1346. Robert Stewart (later King Robert II) held the title from 1357 and it remained active under Stewarts until 1437 when it lapsed. The title of Duke of Cumberland and Strathearn was created in 1766 for Prince Henry (1745 to 1790) who died without issue. The title Duke of Kent and Strathearn was created for Prince Edward Augustus, the father of Queen Victoria. The title of Duke of Connaught and Strathearn was created for Prince Arthur, third son of Queen Victoria. The title lapsed with the (mysterious) death of Prince Arthur’s grandson Alastair in Ottawa in 1943.
The barony of Carrickfergus is in Antrim, Northern Ireland. The title of Baron of Ennishowen and Carrickfergus was created in 1841 for George Chichester to get him into the House of Lords after he failed to win one of the two seats for Belfast in the elections. He succeeded his father as third Marquess of Donegall three years later. The baronial title lapsed with his death.
Knowing where the titles came from is one thing. Using them correctly is quite another one. Even some British commentators got it wrong when describing the newlywed Catherine Middleton as Princess Catherine. She is not. Marriage to a Prince does not automatically make you a Princess. There are certain rules to titles, and they are not too difficult to understand.
Titles are conferred at the pleasure of the monarch, and the Queen followed the rule book to the letter when conferring titles on Prince William before his wedding to Kate Middleton. She wasn't impelled to do so because she is old fashioned; she was governed by good sense and to keep peace in the family. Rules, though seemingly dusty and outdated, sometimes serve a good cause.
If the Queen hadn't conferred any titles on Prince William, Catherine would have become Princess William of Wales; my favorite example in that direction is Princess Pushy. She is not a Princess and therefore correctly addressed as Princess Michael of Kent. The Queen didn't confer any titles on Michael and Marie-Christine therefore has to make do with the courtesy address.
But it has been a tradition for quite some time to confer titles upon important members of the Royal Family when they marry and are not already holding titles of their own; and in case of senior members, the titles conferred reflect the United Kingdom with its composite nations.
William has been made Duke of Cambridge (in England), Earl of Strathearn (in Scotland), and Baron Carrickfergus (in Northern Ireland). That makes Catherine a Duchess, a Countess, and a Baroness, but it doesn't make her a Princess. Because the title Prince of William's is a personal title of descent, it does not confer upon his wife.
Why then was Diana (correctly) addressed as a Princess? Because Prince Charles not only had his personal title (as a Prince of the United Kingdom) but also that of Prince of Wales, a territorial title for a Principality. And as the wife of the Prince of Wales, Diana acquired the title Princess of Wales. The accolade HRH (Her Royal Highness) was accorded and withdrawn at the pleasure of the Queen, though. Catherine was not left out by the Queen in that department; she conferred a HRH (Her Royal Highness) on her, and no, that did't come to her by marriage.
Funnily enough, nobody ever proposed the title of Princess for Camilla who likes to style herself as Duchess of Cornwall though she really is Princess of Wales as well no matter how you look at it. This also means that she will be Queen Consort should Charles ever ascend the throne, no matter what title she chooses to be addressed by. And yes, I am a bit tired of all these discussions about whether she will become Queen or not, that decision was taken when she married Prince Charles and can't be changed anymore except by a change to the constitution.
I suppose meanwhile we all get used to the Duchess of Cambridge, on the other hand. It will also help anyone in ferreting out the articles written by people in the know (using the correct titles) and the mere gossip mongers (who will probably refer to Princess Catherine as it sounds so much more snazzy).