However, there are a few that are actually just urban myths, one of which concerns jewellery.
It’s widely believed that the royals do not wear diamonds in the daytime, but Grant Harrold, also known as The Royal Butler, has told us that this simply isn’t true.
In fact, the etiquette expert has revealed “the more significant event, the more likely you are to see them wearing diamonds – specifically when there is a reason.”
While Kate is unlikely to wear pieces such as the Queen’s dazzling Nizam of Hyderabad necklace to a daytime engagement, diamond earrings and delicate bracelets are not off-limits.
The Queen and Kate also regularly wear bejewelled brooches, particularly on overseas visits or royal tours.
The Duchess of Cambridge has sported the diamond encrusted Maple Leaf brooch, which originally belonged to the Queen Mother, on both her royal tours of Canada in 2011 and 2016, as a tribute to the host nation.
Grant added that jewellery would be chosen to reflect the tone of the event and naturally, statement pieces would be kept for evening engagements.
He said: “Kate and Meghan often wear diamond earrings and both their engagement rings, of course, contain precious stones.”
But there is one rule, Grant explained, that all royals must follow – diamond tiaras can only be worn after 6pm.
They are traditionally worn for evening events and are typically reserved for married women.
Before marrying into the royal family, we never saw Kate or Meghan wearing a tiara in public.
Grant explained: “For married ladies, it was a sign of status and would show you were taken and not looking for a husband.
“For the gentleman, it was a clear sign not to make advances toward the lady in question.”
The Duchess of Cambridge wore the Cartier Halo tiara on her wedding day in April 2011, while the Duchess of Sussex donned Queen Mary’s diamond bandeau headpiece when she wed Prince Harry in May earlier this year.
Follow us on Instagram and Facebook for non-stop inspiration delivered fresh to your feed, every day. For Twitter updates, follow @YahooStyleUK.
Read more from Yahoo Style UK: