Kate Middleton has been known as The Duchess of Cambridge since she married Prince William in 2011.
But her title will likely change when the Queen dies, as The Duke and Duchess of Cambridge move up in the line of succession.
At the moment, Prince Charles is the Prince of Wales: the title which is traditionally given to the heir to the throne.
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Charles is the 21st man to be given it.
Diana became the Princess of Wales after she married Charles in 1981, the Mirror reports.
After the Queen's death and after Prince Charles becomes king, William will become the Prince of Wales as he is first in line while Kate will likely be given the title of Princess of Wales.
Camilla turned down the title out of respect for Diana - and, likely, fear of a public backlash.
However, when Charles takes over, the current Duchess of Cambridge is expected to accept the official title in honour of Prince William's mother, according to British Travel Heritage.
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