sarah ferguson

Difference Between a Duchess and a Princess?
Difference Between a Duchess and a Princess?
Difference Between a Duchess and a Princess?
I've been reading so much about Prince William and Kate Middleton's  wedding coming up at the end of the month.  Once Kate, a commoner, says "I do", she will be an official princess.  I also read that Duchess Sarah Ferguson was NOT on the guest list, while her two daughters, also with the title princesses was.  What?  What makes a princess different from a Duchess ?