It all started with Queen Victoria

So much goes back to Queen Victoria, for good and for ill.

Her descendants have sat on the thrones of 10 European countries - and still do in five cases.

There are trails of her genes throughout, it seems, about 90 per cent of all European royal lines, including the royal families of Spain, Denmark, Norway, Greece, Italy, and of Austria (Tuscany line), Hesse and Rhine (all three lines, of course).

Not to mention the likes of Schleswig-Holstein-Augestenburg, and who could forget, Schleswig-Holstein-Glucksburg.

You've always got to invite the rellies (sigh). But if Victoria's genes kept many of these dynasties alive, they also very nearly wiped them out.

It was Victoria's haemophilia gene, passed through her granddaughter to the Russian court, which led to Grigory Rasputin being called in to treat Alexei, ailing son of the last empress - Rasputin's subsequent influence over Alexandra led to the fall of the Romanovs, the end of the Russian imperial family and then their wholesale slaughter in 1918.

Across Europe, lesser royal families began to topple: the state of European monarchies one century ago was, as the history books say, parlous; in Scotland, they say their coats were hanging on a very shaky peg.

Today, as much of the world gears up to celebrate, or at least watch, the forthcoming Westminster nuptials, and William's continental relatives clutch those invites with varying degrees of decorum, a surprising number of European royal families have survived, even blossomed.

They've survived war, tragedy and nearby revolutions. They've survived scandal and the rise of the tabloid press, and of tabloid thinking. They've even managed to mate, properly, for once.

After awhile experimenting with inbreeding, they presumably grew fed up having to hide away unlucky sons born with seven buttocks and the head of an ant, and married out.

Recent European royal marriages have been to the likes of gym instructors, personal trainers or, simply, a nice girl met on a beach.

"Actually, quite of a few of the European royals today seem to be marrying PR girls," says Morris Bierbrier, formerly co-editor of Debrett's.

"Which makes a certain sense. They're often quite pretty and know how to present themselves but, more importantly, they know how to sell, how to display the couple and the monarchy, and that's pretty much all it's about these days."

So they'll survive?

"Oh, yes. Most European royals will surely still be with us in another 100 years. They've adapted, you see ... Most are treated with far more respect than we afford our own royal family."

Why? Do they live less grandly, or less openly grandly?

"Well, no. It's not as if all European royalty is meek, living quietly in a shack, the so-called bicycling monarchs. Quite the opposite; just like ours, they've got huge palaces bang in the middle of the capital.

"It's partly - actually quite a lot - to do with press privacy laws and, more recently, this disillusion with almost all politicians that has strengthened the case against presidencies.

"With most European royals, there seems to have grown up a kind of unspoken contract which doesn't exist in Britain - a decent mix of what some would call deference, but you could simply think of as respect.

"The Queen of Denmark can go shopping in a department store and nobody bats an eyelid. And European royalty, in return, tends to treat the more intrusive press in a different way from here [in the UK].

"There was something awhile ago about the King of Sweden's mistresses, quite a fuss, but the court chose, far from confirming or denying what was being alleged, to consider it no one's business, far least the press. They simply ... ignored it."

And thus, in just 90 years, from the brink of extinction, the Euro-royals have in general - Monaco, after Britain, always seems to be the exception - forged a truce with their public and their press, which Prince Charles would surely kill for. Partly, it seems, the Euros have managed this because we simply won't recognise them when they turn up at the abbey - apart from the BBC man with the cheat list. We don't recognise them (although you will doubtless be grateful to now possess this information thanks to today's guide) and for sure the Americans don't.

Amy Odell, who edits the New York Magazine's fashion blog, The Cut, is charmingly unknowledgeable.

"I mean, we're all wild about Kate. I'll be up at dawn to watch. But the other European royals? I don't think so. I mean, sometimes. There was one, recently, a princess got married, from I think Sweden, whatever.

"That was kind of nice. But just for one day. We kind of forgot which country the next day. There's a bit of interest in Princess Letizia of Spain, and in Carla Bruni, though I suppose she's not actually royalty."

It is as hard to generalise about European royals as it is about any group of humans. There are ones who do tremendous good works, often quietly.

Then there are what you could call the Ferrero-Rocher royals, often the ones dispossessed of kingdoms - officially, a royal family is still classed as one ruling a sovereign country at the time of the Congress of Vienna in 1815, whether or not said country has ceased to exist. There are 21 of these.

Many of these people live in New York and feature in lists of eligibles.

But who, I ask Dr Bierbrier, is top dog, head honcho, grand fromage, among the European royals? The British royal family, which basically bred them all?

"I don't know, honestly. There are some monarchies which are grander, stand on ceremony, more than here. Our Queen is friendlier with some more than others. Often, it depends on just whether they're a close relative. She's very good friends with the Queen of Denmark, who loves to leave the palace and go shopping in Harrods - because she can, because no one knows.

"But it's a nebulous thing, the grandness - mysterious, silent, unknowable. I always remember the tale of Victoria entertaining Napoleon III. It was noted, by those who note, that at supper Victoria and Albert simply walked to the table and sat without looking down, so certain were they that the chairs would have been pushed in behind them. Napoleon and his wife looked down."

Don't look down, Kate.

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