That is, the lady formerly known as Kate Middleton. Wikipedia lists her full name as just being ‘Catherine Elizabeth’, but - they are from the house of Windsor, isn’t that equivalent to having your last name be Windsor?
She would have the surname of her father-in-law, who is the Prince of Wales. However, neither he, nor her husband, needs a surname, since they are royal princes. If they would ever need a surname, it would be Mountbatten-Windsor.
Right, sure, everybody knows who they are, but if they have children and enroll them in school, surely they need a last name for something. Anyway, thanks!
I’ve read that the four children of Queen Elizabeth, when they were in school and, where applicable, when they were in the military, were called “Windsor” by their schoolmates, by their teachers, and by their superior officers. It’s common for students at public schools in the U.K. to call their schoolmates by their last names in many circumstances where it would be more common to call them by their first names in the U.S.
Plenty of people don’t have last names. I don’t think anyone is going to really force you to make one up. You might need something to type to make it through some computer entry forms, but this can be a workaround, much like how different systems have different workaround for people with apostrophes in their names or two last names or whatever.
As a data point, I had a friend whose only name was Omar, and he went to the US on business. The embassy had no idea what to do, but they eventually filled out his forms as “Omar Omar.” But obviously “Omar” didn’t become his official last name or anything- it was just a workaround.
I’ve seen examples of the Queen’s descendants using Windsor, Mountbatten-Windsor (I think at least one of the Queen’s children used it when they got married, despite not being a Mountbatten-Windsor according to the order-in-council creating the name), the place name from their or their parent’s peerage, or nothing* as a surname. They seem pretty relaxed about the whole thing, really. I don’t think anything legally binds her to using one surname and one surname only, though, so it’s not like she has to have just one specific name.
*When Prince Charles got married for the second time, the certificate asked for his “name and surname.” Whoever filled the form in just put “His Royal Highness Prince Charles Philip Arthur George, the Prince of Wales.” His title was capitalized, and so was “Parker Bowles” in Camilla’s name, so I suppose he used “the Prince of Wales” as a surname then and not nothing.
There is already a standard convention for the situation where a person has one or more “missing” names. NFN, NMN, NLN are the abbreviations for No First Name, No Middle Name, No Last Name.
Titled persons, especially in the Royal Family, use their title in lieu of surname. Prince Charles’s legal signature, for example, is “Charles P. (for Princeps) Wales.” Andrew’s is “Andrew York.”
As I noted in a related thread last week, the Royal House continues to be known as the House of Windsor, the fact that the Queen is the wife of a Duke whose surname is Mountbatten being irrelevant to the throne. However, their children and grandchildren, when in need of a surname, prior to 1960 used “Windsor” and since then are to use “Mountbatten-Windsor.” Thus, the marriage register, which is purely democratic in seeking given name and surname, not title, shows that Catherine née Middleton married William Mountbatten-Windor. But she now is known as Catherine, Duchess of Cambridge.
Middleton-Windsor if she was American. We do love our hyphened names these days. 
Why did they have to take away the poor woman’s first name too? One day she’s Kate and the next Catherine. I know that’s what “she signed up for”. IIRC Kate separated several times from William while they were dating. She knew the hell Diana went through and was unsure about this Duchess of Cambridge gig. It really does take away any identity she had previously. She was a college grad and a successful business owner.
To build on this–
Why is everybody referring to her as “Duchess”? She’s a PRINCESS, for pete’s sake! She married a PRINCE! Why in blazes would people routinely call her by a lesser title?
I especially don’t understand it because it’s opposite to recent precedent. For instance, Queen Elizabeth’s husband has two main titles, “Duke of Edinburgh” and “Prince Philip.” But he’s usually called Prince Philip, because obviously a prince is higher-ranking than a duke. So why are all these idiots using “Catherine, Duchess of Cambridge” instead of “Princess Catherine”?
I imagine that “Princess William” would sound strange to a lot of people. And dukes are peers of the realm; princes with no peerage dignity are, strictly speaking, commoners, so I’m not sure it’s quite as much of a “lesser title” as you seem to think.
But he was widely known as “Prince Philip” before his marriage, and he was in fact a prince of Greece and Denmark until 1947. He was even colloquially called Prince Philip when he was not properly styled as such, as he was not a prince of any country between 1947 and 1957, when he was made a Prince of the United Kingdom by his wife.
Because “Princess Catherine” is wrong.
When Charles becomes king, will his signature become “Charles K. England?”
I seem to recall that the royals chose the name Windsor (from one of their castles) because of anti-German sentiment in the First World War. Their real surname was Saxe-Coburg-Gotha. For the same reason the Battenbergs (Prince Philip’s family) anglicized their surname to Mountbatten.
It will probably be “George R”.
“Charles R.”, unless he takes another regnal name. He won’t be King of England – the last King of England died 310 years ago.
Wouldn’t “Charles K. U.K.” be more correct?
Saxe-Coburg-Gotha was the house name, but the surname was enough of a mystery that it had to be researched when Queen Victoria wanted to know her husband’s surname (the College of Arms found that it was Wettin, but IIRC not everybody agreed that he even had a surname, let alone that it was Wettin).
His (most recent) marriage register was just signed “Charles.” I saw “Charles P” once on a document he signed as a Counsellor of State, but never “Charles P. Wales.”
Charles R. (for Rex). It won’t be “of England” because he’s not King of England (other than in the same way he’s King of Prince Edward Island or Tasmania)… he’ll be (1) King of the United Kingdom of Great Britain and Northern Ireland; (2) King of Canada; (3) King of Australia; (4) King of New Zealand; (5) King of the Bahamas; (6) King of St. Vincent and the Grenadines; and about 15 other titles. But not “of England” or “of Scots” – those two titles haven’t been around since 1707.
Because she’s not a princess, at least, not in her own name. She’s only one in her husband’s name. (As someone mentioned, “Princess William”.) Marrying a prince doesn’t automatically make you a princess – the Queen would have to make her one, I believe. (Letters Patent, I think?)
Her official title is HRH, Catherine, the Duchess of Cambridge. A royal peerage is not a “lesser title”, in any way, shape, or form. Especially being the wife of the heir presumptive. (In fact, I believe in many countries of Europe dukes out rank princes, if I’m not mistaken. I know grand dukes do)
British Prince
British Princess