Something has happened, Mr. Kidd, and if you are not sitting down now, one suggests you do so. One has heard that some Americans — out of innocent ignorance, surely — have referred to Prince William not as “His Royal Highness” but as “His Majesty.”

“Goodness me.”

Charles Kidd gives a sharp little gasp as he absorbs this offense. “Oh dear,” he says.

Kidd is the editor of Debrett’s, which is the genealogical guide to the living British aristocracy — every duke and marquess and earl and count and viscount and baronet (though baronets are not a part of the peerage but rather the baronetage, which is different). A print volume of Debrett’s costs about $500 and is approximately the size of a Cadillac. It is sort of like a very fancy Facebook.

We are T-minus one month to the wedding of Prince William and Catherine Middleton. When he marries, if he follows the current royal tradition, William might acquire a brand new title. His grandmother, Queen Elizabeth II, will give it to him. This is how, for example, Prince Andrew became the Duke of York when he married Fergie. And so, in preparation for this auspicious event, a crash course. A crash course called Understanding the British Titles, which is really about understanding the British, and our general sense of inferiority toward them.

“Well,” Kidd says efficiently. “Let’s begin with a quick rundown of who’s who, shall we? The queen is Her Majesty the Queen. Her eldest son is His Royal Highness the Prince of Wales. His eldest son is His Royal Highness Prince William of Wales. Her husband is Prince Philip, Duke of Edinburgh.”

But if you look up Prince Philip on the royal family’s official Web site, it appears that he is also Baron Greenwich and the Earl of Merioneth, only Merionethshire doesn’t appear to exist anymore, and —

Oh dear indeed.

Mr. Kidd, it appears that this is getting all too confusing. Perhaps it is necessary to begin with a more remedial lesson. Perhaps one must call someone who can break all this down into simple words.

Perhaps one must call . . . an American.

“Any time you see ‘peerage,’ it means nobility,” explains Washington-based Kitty Kelley, author of “The Royals.” “There are two kinds of peerage: the life peers, whose titles die with them, and the hereditary peers. Hereditary means you were born into the lucky sperm club.”

The royal family’s club is naturally the luckiest of lucky. Out of dozens of existing duchys, a few are typically reserved for royals.

The trouble is, most of those are already in use. The title of Duke of Gloucester belongs to Prince Richard, the queen’s first cousin, who inherited it from his father. Another cousin, Prince Edward, is the Duke of Kent, a title he also inherited from his father. The queen’s youngest son, also Prince Edward, is the Earl of Wessex, a title he inherited from — well, apparently from no place, because it hadn’t been in use for nearly a millennium. The London Telegraph reported that Edward was a fan of the film “Shakespeare in Love,” in which Colin Firth plays a fictional earl of Wessex.

“Oh, honey, they can just make them up,” Kelly says. “Catherine Middleton is going to have titles heaped on her. Just heeeeaped on her,” to make up for the fact that she wasn’t born with any herself.

Example: When Princess Margaret married the commoner photographer Antony Armstrong-Jones, he was given the title of the 1st Earl of Snowdon, and started going simply by “Snowdon,” much like Madonna is just “Madonna.”

Example: Princess Royal, the title currently held by the queen’s daughter, Anne, was invented in the 17th century when Queen Henrietta Maria, wife of King Charles I, decided it was neat how the oldest daughters of French kings were referred to as “Madame Royale.”

All of this — the naming and renaming and dukes and earls, it all begins to seem like some sort of diversionary tactic. Look, the emperor has no titles!

“Americans get very confused when a prince becomes a duke,” Kidd says wearily. “They think it’s a demotion. But really it’s just — if you’d just describe it as a tradition. A sign of recognition.”

One would have assumed the future king of England would have all the recognition he needed, being, after all, the future king of England.

There are Americans who know all of this stuff, who own copies of Debrett’s or its competitor Burke’s, who go on message boards and pooh-pooh the people who refer to Princess Diana, when there was never a Princess Diana.While she and Prince Charles were married, her correct title was Her Royal Highness the Princess of Wales; after their divorce, she was styled Diana, Princess of Wales. (Right now — right now — these same people are annoyed that I didn’t note that Prince Edward might have chosen the title of Earl of Wessex because he will likely also inherit the dukedom of Edinburgh from his dad.)

Why do they know these things? Is it a way of separating themselves from the rest of the heathenish Americans? Or maybe the people who pride themselves on understanding the peerage are just the people who always stand on the right, walk on the left, and remember to rotate their winter and summer clothes in a timely manner. Maybe this form of Anglophilia represents an appreciation for a world that is either correct or incorrect, ordered or not.

The British may be Americans’ kissing cousins, language- and culture-wise, but this coziness means that when differences do arise they are all the more shocking. Everything seems to be normal, until you are in a Scottish grocery store and you realize they do not sell Oreos.

The peerage seems to represent the frank resignation particular to Brits. Yes, it says. There are people who are just born lucky. There are social spheres you can never reach. This is true in the United States, too, but we are not crass enough to admit it.

British titles are a way of declaring that there is wealth, and there is standing, but only one can be acquired.

Or — wait, a discovery! ­ is a site that auctions off Scottish titles, starting at around 75,000 British pounds, presumably so that the title-rich, cash-poor nobility can keep themselves afloat. Currently two titles are available: The barony of Denny, which dates back to the 16th century, and the barony of Kerse, from 1390.

But back to William and Kate.

If he takes no new title, he will continue to be Prince William of Wales, and his wife will be Her Royal Highness Princess William of Wales. Of course, people will mess up and call her Princess Catherine. (Speaking of Middleton and titles — her name is Catherine. She goes by Kate. Does she not realize that she has violated the sacred C/K non-crossover principle upheld by Catherines, Katherines and Kathryns? Please refer to Cate Blanchett and Kate Beckinsale.)

According to a Clarence House representative, “It’s a decision that lies only with Her Majesty the Queen. In the past, the announcement has not been made until the day of the wedding.”

If the queen does choose to bestow on her grandson a new title, there are a few options.

The Duke of Clarence has a nice ring to it, but the most recent person to hold that title was Prince Albert Victor, Queen Victoria’s grandson. The prince, who went by “Eddy,” died before reaching age 30 — which was still enough time for people to speculate that he was the serial killer known as Jack the Ripper.

The Duke of Albany used to be in rotation, but now it’s kind of “in limbo,” writes Ken Cuthbertson, who blogs for the Web site Unofficial Royalty. It had belonged to another of Queen Victoria’s grandsons, but he supported the German Kaiser during World War I and later became a Nazi. It was all very awkward; the title was revoked after the 1917 passage of the Titles Deprivation Act, which punished the four members of the peerage it claimed sided with the wrong guys during the war. These included the 12th Viscount Taaffe, who was also Baron of Ballymote.

Currently, the front-runners among the people who watch this sort of thing are the titles of Duke of Cambridge or Duke of Sussex.

There is nothing objectionable about either of these titles, assures Kidd, who also assures that he knows this can get rather confusing. At the offices of Debrett’s, he says, “we have to look things up, too,” but one gets the distinct impression he is just being kind.