When Kate Middleton marries Prince William she will not become Princess Catherine. She may be known as Princess Kate or Princess Catherine but this will not be her formal title. Diana, Princess of Wales was known as Princess Diana but this was not correct.
In calling her Princess William, you are right. Her formal title will be HRH Princess William of Wales. Like Princess Michael, Kate will take her title from her husband. She will not be a princess in her own right - like Princess Beatrice or Princess Anne. She is a princess by virtue of her marriage only. This is the reason that Marie-Christine is known as Princess Michael of Kent and not as Princess Marie-Christine of Kent.
The Queen could take the step of creating Kate a princess in her own right, but it's not likely - never say never though! Should she do this then I presume Marie Christine and Sophie, Prince Edward's wife, would want the same designation as well.
However, if Prince William is granted a dukedom upon his marriage then Kate will be known as HRH Catherine, Duchess of Clarence (or another available dukedom). If Prince William does not receive a dukedom (and there are reports that say he doesn't want one) then Kate will be known as HRH Princess William of Wales. In time, when William becomes king, she will be known as Queen Catherine.
Although this title is correct, Princess William sounds odd to peoples ears, which is probably the reason why people will call her Princess Kate/Catherine. Let's hope he accepts a dukedom!
© Marilyn Braun 2011
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