Princess Diana's loyal linage is threw her decent of Charles II. He had many illegitimate children and Diana is a descendant of one of his sons. Interestingly Camilla Parker-Bowles is a descendant of another one of the illegitimate children of Charles II.
The royal family
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She became a Princess when she married His Royal Highness The Prince Charles.
The attached link leads to an article with a photo and description of Princess Diana's wedding cake.
The bad point of Princess Diana was thatsheto had an affair with her royal butler Paul Burrel.
She married into the royal family.
When she was born, as the daughter of a viscount, she was called the Hon Diana Frances Spencer, and when her father inherited the earldom she became Lady Diana Frances Spencer: she was born an aristocrat but a non-Royal and therefore a commoner. When she married Prince Charles, she was called Her Royal Highness Diana, Princess of Wales. When she divorced Prince Charles, she became Diana, Princess of Wales.
Diana, the Princess of Wales, died in France in 1997. She was killed in a car crash in Paris on August 31st.
Princess Diana
Diana, Princess of Wales
Yes, Diana was a commoner, but she was hardly the last one to marry a British royal. That would be Princess Beatrice's husband, Eduardo Mapelli Mozzi. She is royal; he is not.
Princess Diana of Wales