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It's not something we like to think about - but one day, the Queen will die.
And while the most obvious changes when that happens will be for Prince Charles and his wife Camilla, who will become king and queen consort, there are also going to be some big changes in the Cambridge household.
Although Prince William and Kate have become key figures in the Royal Family over the last few years, they will have more responsibility when Charles becomes king, and they will be given new titles to match.
When Charles dies, William will become heir to the throne, which means he will inherit the Duchy of Cornwall, a parcel of land across the country, totalling 135,000 acres, which exists to provide an income for him outside of the Sovereign Grant.
William will inherit Charles's title of the Duke of Cornwall, which will make Kate the Duchess of Cornwall.
That title is currently used by Camilla.
But while Camilla, 73, only uses the title of Duchess of Cornwall, Kate will have options, and will probably end up with a very long name indeed.
She's currently the Duchess of Cambridge, after the title was bestowed on her as a gift by the Queen.
She could still use that, as she won't lose that title, so she could be HRH Catherine, the Duchess of Cornwall and Cambridge.
Together William, 38, and Kate, 39, would be Their Royal Highnesses, the Duke and Duchess of Cornwall and Cambridge.
That's not all - as William could inherit another important title.
The title of the Prince of Wales merges with the crown on accession, which means that Charles will have to recreate it if he wants his son to be the new Prince of Wales.
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The Queen bestowed the title of the Prince of Wales on Charles in July 1958 when he was nine.
He was invested as the Prince of Wales in a ceremony in Caernarfon Castle in 1969.
Charles, now 72, has maintained strong links with Wales through his life, including owning his own private home there. William's links are less strong - he was called William Wales while he was in the armed forces, and he and Kate lived on Anglesey in the early years of their marriage.
But they don't currently have a home there, and spend their weekends and holidays in Anmer Hall in Norfolk.
If Charles chooses to make William the new Prince of Wales, it would make Kate Princess of Wales.
That title has not been used by Camilla through her marriage to Charles because of its association with the late Diana, but Kate is sure to have the public support to use it.
It would make her HRH Catherine, Princess of Wales, Duchess of Cornwall and Cambridge.
The title of Duchess of Cambridge is not the only one Kate was given when she married William.
She has different titles to use in other parts of the country. In Scotland, she is the Countess of Strathearn and in Northern Ireland, she is Baroness Carrickfergus.
One day much further in the future Kate will be the queen consort.
She will likely be referred to as Queen Kate or Queen Catherine, though she is technically the consort because she has married into the Royal Family, rather than been born into it.
And she could one day be the Queen Mother, if her husband Prince William dies before her, as her son Prince George would then become king.
But that's certainly not happening any time soon.