Kate Middleton and Meghan Markle should curtsy to relatives who are royal blood even if they're not as close to the throne, an expert says.
At royal gatherings everyone should curtsy before the Queen, but the two duchesses are meant to formally greet princesses from birth, like Princesses Anne, Beatrice and Eugenie.
The ancient rule of precedence was first rolled out by Tudor Queen Elizabeth I in 1595, when she decided that her blood family outranked all others.
British aristocracy expert Wendy Bosberry-Scott said women who marry in are viewed as having lower status than their full-royal blood counterparts.
Bosberry-Scott, editor at etiquette publisher Debrett's, told People: "Women of the royal family who marry in like Kate and Meghan gain their titles by marriage and therefore are of a lower rank than those born into their titles."
A Debrett's spokesperson added: "Essentially, the hierarchy exists to ensure that the princesses who are born with royal blood, like Eugenie and Beatrice, aren't kind of pushed to one side.
"As far as the public is concerned, the most visible representation of this tends to be the order in which they might stand at a public event, or the order in which they might enter or leave a room, or who curtsies to whom."
Kate and Meghan's position in the family is further complicated by the traditions of the monarchy.
Almost always, men are given precedence over women - apart from the Queen, 95.
So the status of the two duchesses is tied to their husbands, Princes WIlliam, 39, and Harry, 37.
Bosberry-Scott explains: "When the wives are with their husbands, their status is elevated to reflect the fact that the men are present.
"As the men have a higher rank than the princesses (Anne, Eugenie, and Beatrice) they have precedence, and this higher rank is reflected on their wives, so they are then moved ahead."
Meghan should also technically curtsy to Kate because her husband, William, is the second heir to the throne, which ranks him higher than Harry.
But though Kate "outranks her", the Debrett's spokesperson says, the curtsy would only be performed at a public, formal meeting, which has not yet been seen.
The ancient rule is again complicated by Prince Charles's wife Camilla, the Duchess of Cornwall.
As she's married to the heir of thee throne, she ranks higher than Kate, Meghan, and all her royal nieces even if unaccompanied by the prince.
Camilla doesn't use her title, the Princess of Wales, because it is so linked to the late Princess Diana.
But she still has a higher status, which is "being shown more and more" as the "Queen has moved to place Camilla in precedence over her granddaughters in recent years" to show the public she is the wife of the man who will next be king.
Her rank still depends on Charles's presence as she technically sits below the Queen's cousin when he's not around.
As wives of the two heirs, Camilla and Kate, will gradually begin to fill a higher regal status as their husbands get closer to the throne.
Eventually, all other royal relatives will have to curtsy to them - even if they aren't full-blooded royals.
One indicator of Camilla's trajectory to the top spot was revealed last month, when it emerged that she will be central in the Queen's Platinum Jubilee Celebrations next year.
The Duchess of Cornwall will lead the "Big Jubilee Lunch" on the final day of the special four-day Bank Holiday Weekend to celebrate the monarch's 70-year reign.
The lunch is central to the celebration and a nationwide event where Brit families and friends will meet up for street parties and sit-down meals to honour Elizabeth II, the longest-reigning monarch in British history.
