Did you know that tiaras are reserved for married women or members of the Royal Family? This is why the Duchess of Cambridge wouldn't have been seen wearing one before her wedding to Prince William. According to Diana Mather, tiaras started being worn at formal events because "hats are never worn indoors after 6 p.m." She adds, "Flashy diamonds and tiaras are not worn during the day, and only married ladies wear tiaras. For married ladies, it was a sign of status and would show you were taken and not looking for a husband. For the gentleman it was a clear sign not to make advances toward the lady in question."
Now that she is married, Kate has her pick of all the royal headwear, which she gets to use during formal events such as state dinners.