Royal quiz time: What is (probably) the most closely guarded secret in the royal family?
Is it whether there is any credence to the long-standing but unsubstantiated rumours that Prince Philip's zipper got quite the workout in his younger days?
Is it just quite how sympathetic or not the family was to Hitler in the 1930s given in 2015 footage was unearthed showing then Prince Edward, the future Edward VIII, teaching the Queen Mother, Princesses Elizabeth and Margaret the Nazi salute?
Or is it why the Queen mysteriously intervened at the 11th hour to prevent Diana, Princess of Wales' former butler Paul Burrell standing trial in 2002?
Nope. (Well, at least in my opinion.)
Rather, I'd wager, it's the true extent of the house of Windsor's wealth.
Because, not a soul outside of the Queen's family has any real understanding of the full breadth of the clan's personal holdings and just how much money they have socked away. (Their family wealth that is, and not the places, art and priceless jewellery that are the property of the Crown.)
This week the question of the royal house's finances was shunted back into the uncomfortable spotlight courtesy of recalcitrant former Firm 'employees' Harry and Meghan, Duke and Duchess of Sussex.
It has been a seismic, dizzying seven days which has seen them take a metaphorical sledgehammer to the mystique of the whole royal game, raising charges of shocking racism and cruelty inside palace walls. Among the issues raised was how the family supports various members.
Harry, speaking to Oprah Winfrey, revealed that in the wake of he and Meghan's quitting as working members of the royal house in January last year, "My family literally cut me off financially."
(Prior to Megxit, Prince Charles, had supported the couple to the tune of about $4.86 million annually according to the Duchy of Cornwall's most recent accounts.)
He later added, "The biggest concern was while we were in Canada, in someone else's house, I then got told, short notice, that security was going to be removed."
Saving the day financially, according to Harry, was the money bequeathed to him by Diana, Princess of Wales.
"I've got what my mum left me and without that, we would not have been able to do this. I think she saw it coming," he said.
(This logic falls down a tad considering that Diana is believed to have left an equal sum to William. Did she too foresee he would need her money to help pay school fees and keep his wife in a never-ending series of prosaic floral dresses?)
The overall image, cast by Harry, was of the couple (and their baby son Archie) essentially cast out, suddenly severed from the royal teat and left with nothing to live on but Diana's cash, their wits, celebrity connections and deal-making nous.
However, as the house of Windsor finds itself stricken with a bad case of PTSID (Post-traumatic Sussex interview disorder) reports have started to emerge suggesting that there is another side to this money scuffle.
So did Charles punitively cut his son off in fit of pique? Or was he sick of shelling out fistfuls or crisp readies to his son and daughter-in-law?
Speaking to The Telegraph, a "well-placed source" said that "like any parent, Charles did get fed up with the constant calls from Harry for more money".
"He ploughed a lot into the wedding and the refurbishment of Frogmore Cottage (the couple's Windsor home) and did his utmost to make them feel financially supported but when they said they were upping sticks, they asked for even more," the source said.
"If he was less inclined to take calls, it might be because he didn't want to be treated like a cash dispenser. I think a lot of parents will be able to identify with that."
(God, I hope my mum is not reading this.)
Any notion that when Harry and Meghan touched down in LA in March last year they were on Struggle Street, two former HRHs channelling Blanche Dubois and having to rely on the kindness of strangers (go on, insert a swoon here if you fancy) seems a bit rich.
At the point the duo was unpacking their (I'm guessing) his'n'hers meditation stools inside the Beverly Hills mansion lent to them by Hollywood titan Tyler Perry, reports suggest that at that point, they had millions, if not tens of millions of dollars, in their bank accounts.
When the princess was tragically killed in Paris in 1997, she left behind £12,966,022 ($25.2 million) which was reduced to £8,502,330 ($16.5 million) once various death duties had been paid. According to The Mirror, "years of clever investments by royal advisers meant the fortune had swelled to more than $36 million by the time it came back to Harry and his brother William."
The Times this week quoted a more conservative figure, reporting that Duke had inherited an estimated $13.9 million from the late princess' estate, and that "his mother's legacy may have grown significantly before he was entitled to take control of it at the age of 30".
While previously a representative of Harry's confirmed to Forbes that he did not receive an inheritance from his great-grandmother the Queen Mother who left behind a mysteriously acquired $194 million fortune when she headed to great big gin bar in the sky in 2002, this week The Times reported that she had in fact left him $5.8 million.
It is also "highly likely," according to author David McClure who is the author of Royal Legacy, an investigation into the royal finances, that in addition the Duke is also the recipient of income from other undisclosed family trusts, TheTimes reports.
Elsewhere, Norman Baker who is a member of the Queen's Privy Council, reported in his 2019 investigation into the royal house And What Do You Do? that, "Harry has amassed at least $38.8 million in terms of personal wealth."
Meghan, meanwhile, is reported to have entered royal life with somewhere between $2.8 million and $5.8 million to her name, largely thanks to her role in Suits.
Even leaving aside the money which the Queen Mother may or may not have bequeathed Harry and taking the most conservative figures thrown about, when they started their new shiny lives in the US they had $16.7 million to their names.
Hardly a trifle right?
In mid-June a trust, which shares a mailing address with the offices of Meghan's longtime business manager, purchased the $20.5 million Montecito compound which the family now lives in, a deal which happened more than two months before they announced their Netflix deal at the beginning of September.
All of which makes Harry's indignation at the parental money spout being turned over just that much harder swallower.
Added to which, is there not something a tad unseemly about a 36-year-old grown man having a self-indignant whinge that his 70-something father has turned off the money spigot? Especially at a time when his new homeland, the United States, is in the midst of one of the biggest economic crises in a century?
Suddenly having to pay for one's own private jets might have come as a rude shock but is hardly worthy of any sympathy.
It would be unfair to not point out here that a factor in his money worries came down to the fact that their Stateside flit left the family suddenly having to make their own, pricey arrangements for security.
However, the fact that it seems to have not occurred to either Harry or Meghan that UK taxpayers might not be thrilled to foot the bill for an LA outpost is a bit of a puzzler.
(On that front, here's a quick primer: The royal family is protected by members of Scotland Yard's Royalty and Specialist Protection (RaSP) and therefore all decisions about their protection are made by the London Metropolitan Police and their boss, the Home Secretary.
Currently, only working members of the royal family receive police protection and simply having a title does not entitle a Windsor to a bevy of gun-toting specialist officers. For example, Princesses Beatrice and Eugenie are not protected by taxpayer-funded bodyguards.)
Still, those hardscrabble Sussexes, as we now know, did fine when it came time for them to fend for themselves last year, going on to sign a megawatt deal with Netflix, which some reports have pegged at being worth more than $140 million, and an estimated $58 million deal with Spotify.
What a difference a year makes. Suddenly Daddy's $4.8 million allowance is starting to seem a bit piffling …
Daniela Elser is a royal expert and writer with more than 15 years experience working with a number of Australia's leading media titles.