Meghan Markle at Christmas services, curtseying
Members Of The Royal Family Attend St Mary Magdalene Church In Sandringham KING'S LYNN, ENGLAND - DECEMBER 25: Princess Beatrice, Princess Eugenie, Princess Anne, Princess Royal, Prince Andrew, Duke of York, Prince William, Duke of Cambridge, Prince Philip, Duke of Edinburgh, Catherine, Duchess of Cambridge, Meghan Markle and Prince Harry attend Christmas Day Church service at Church of St Mary Magdalene on December 25, 2017 in King's Lynn, England. Getty Images

Christmas morning, Meghan Markle took part in her newest part of royal protocol: curtsying to the Queen. She attended Christmas morning church services at Sandringham, one of the Queen’s estates. Those services were the site of her first public curtsy.

Curtseying out of respect to the Queen of England is an obvious matter of protocol. But the question of who has to curtsey for whom becomes trickier as you go down the line of succession.

For instance, Markle must always curtsey for her future sister-in-law, Catherine, the Duchess of Cambridge. However, for other women in the royal family, it will depend on the presence of their royal husband. Markle would need to curtsey to Sophie, Countess of Wessex, only when Prince Edward is also in the room.

Perhaps more confusingly are those individuals whom Markle should sometimes curtsey for but sometimes receive a curtsey from. She will normally have to curtsey before Princess Eugenie and "of the royal blood": They were born royal. But if Prince Harry is in the room with her, then the princesses will have to curtsy her instead because she takes Harry's rank, and Harry is the son of Prince Charles, who outranks his younger brother Prince Andrew.

Got that?

For men, Markle’s decision is made much more simple: If he’s a prince, she curtsys.

The rules governing who curtsys to whom are not strictly based on the order of succession, but are rather based on a document from 2005 called the “‘Precedence Of The Royal Family To Be Observed At Court.” It was meant, those who study the royal family say, to keep Princesses Anne and Alexandra above Camilla in seniority at Court given that they had been serving the royal family much longer.

Princesses Anne and Alexandra, both blood princesses, had to curtsy to Charles' first wife, Diana.

The British royalty adopted the curtsy is the 16th century, as a less formal alternative to a full-blown dropping to one’s knees before royalty.