Not sure about this. If Earth blew up and Prince Charles somehow survived then I think it would be odd and redundant to keep referring to him as a prince just because it was a title he used to hold in a previous life.

That doesn't quite equate to the question, that would only count if Prince Charles was ruling Mars then Mars blew up but Earth remained, then that would be the same scenario.

Former POTUS are still accorded the title of "Mr President". Sure, the USA didn't cease to exist, but for them personally it's a "previous life", yet etiquette is that the title persists beyond the post. I don't have a real world example with regards to royalty, but it doesn't seem unreasonable that a similar courtesy would be extended.

There have been several examples among history, where although their respective countries were absorbed by a conquering nation, royalty families maintained their original titles or were granted new ones (usually due to active collaboration with the new regime). So nobility titles can extend themselves further than the existence of the country that creates them.

Also note that Prince Philip is only Prince Philip because Her Majesty Queen Elizabeth II granted him that title on Feb 22nd, 1957, via Letters Patent. Prior to that, he was not Prince Philip, because to marry Her Majesty he had to renounce his previous Greek and Danish titles. (citation) He is not Prince Philip just because he's a member of a princely family.

@TheDarkLord - no, that's precisely and exactly what we do. Retired military, political, regal, academic titles are, precisely, used indefinitely after retirement (or similar changes, siuch as your world-blown-up example!)

@TheDarkLord , I know what you mean when you get "caught in a loop" considering questions like this! One way to think about it ... "Name Titles" such as Professor, General, Major, President ... are just like "Mister" You see? I'm Mr Smith, you're Mr Dark, my daughter is Miss Smith. The title "Mr" is and will always be the "title-thing you put in front of my name". It's absolutely the norm and usual procedure that such titles "continue" ... your various retired uncles (Judge Dark, Major Dark, Doctor Dark) are simple examples! (Quite an overachieving family!)

Former POTUS are often called president, but that is incorrect as President is a title which is singular. Correct titling after Trump leaves office will be Mr Trump, Senator Obama, Governor Bush, Governor Clinton, etc. Incorrect titling abounds, but it is still incorrect.

@J.ChrisCompton the "President" title for former presidents is extremely common usage, and is generally used by current and former presidents themselves. If common usage isn't correct, then what's the source of correctness?

@JustinLardinois History. Assuming what politicians currently is socially correct will often lead you down the wrong path... no matter which side of the aisle they are from.

@delinear In principle it might have been applicable to Queen Elizabeth's uncle, King Edward VIII. They specifically went out of their way to control his access and use of titles and offering arguments his abdication sundered all rights to titles except as granted by the new King (or subsequent monarchs). They allowed him only "His Highness" (and expressly denied the corresponding title to his wife; they wouldn't even let people curtsy to her), and the title of a royal duke specifically to bar him from becoming a member of the House of Commons or speak politically before the House of Lords.

@JohnKugelman I think you will find that a Governor who has not gone on to be President is often referred to as "Governor".

"Leia was adopted ... this makes her a princess." In the UK at least, adopted children do not acquire courtesy titles (like "Princess") from their adoptive parents.

@JohnKugelman +1 Great question! Sorry, I wasn't complete. There are titles that are held in parallel like "Governor", "Senator", "The Honorable" (title for House members, whether they are actually honorable or not), "Judge", etc. The highest title remains with the person until death. A title is higher if fewer people currently hold it - so Governor (50 states have 1 each) outranks Senator (102 of them) outranks The Honorable (hundreds of House members). A singular title passes to the next person exclusively ("President" plus I assume "King", "Queen", "Prince of Wales", etc.)

Why are you placing President in its own category? Where is this singular versus parallel title concept coming from? I feel like this distinction is one you came up with, along with the rule I've never heard of that "a singular title passes to the next person exclusively".

Europe is full of princes whose grandfathers lost their kingdoms a hundred years ago. While their titles have been cut or turned into surnames, the public still likes to address them as "Prince X" and "Princess Y", for fame or shame (usually the latter).

@MartinBonner That's more due to that in the UK there's been very little (zero I think) adoption by the royal family.

@RyanMcDonough I agree there is no adoption in the royal family, but there has in the wider aristocracy, and the rule is very clear: no courtesy titles, and no inheriting titles. (Of course, the monarch is free to make an adopted child a princess by Letters Patent, but it wouldn't be automatic.)

@MartinBonner oh yeah in the real world it's less likely to happen but in Star Wars it did.