A curtsy is a polite gesture which is used to show respect and gratitude, usually to someone of a higher social rank or status. In the Western world, it is the female equivalent of bowing. Dickie Arbiter, the Queen’s former press secretary, said: “Bowing or curtsying when in the presence of a member of the Royal family? Not necessary. Not unless one chooses to. The Queen is very relaxed about it.”
It is still nice to curtsey, especially to Her Majesty, simply out of respect, especially if you’re British or part of the Commonwealth as the Queen is our head of state. If you do choose to curtsey, remember that the modern curtsey involves the knees facing forward rather than sideways. One places their weight on one leg whilst the other sweeps behind and takes most of the bend. Keep your back straight. One should curtsey once when the Queen enters the room or you enter her presence and once when she is about to leave.
Although Kate has no royal blood in her, her marriage to William has secured her a status of higher ranking as she will one day be Queen Consort. However, her background means that she should still show reverence to the blood royals, especially when William is not with her.
She must curtsey to the Queen, Philip, Charles and Camilla with or without William. However, if Charles is absent and Kate is with William, Camilla is outranked by Kate. This is because William outranks Camilla due to his lineage and therefore so does his wife. When Kate is alone with Camilla, she should curtsey to Camilla since she is her senior. That being said, Kate has never been seen curtseying to Charles and Camilla (or other royals minus the Queen & Philip & monarchs from other countries); it is most likely that they told her she does not have to. She gives them a kiss on the cheek instead
The likes of Princesses Anne, Alexandra, Beatrice and Eugenie all outrank Kate when she is on her own as they are blood princesses. But when Kate is with William, she outranks them as her husband is second in line to the throne. Despite all this, more often than not the family tend to save curtseying for the Queen. The Queen’s granddaughter Zara Philips explained that they keep it up around her because “she’s from an era where that was how it was done.