Why Kate Middleton Won't Be Called Princess Catherine

The reason for this, is that she wasn't born with royal blood in her veins.

Some of the royals -- Princess Margaret, Princess Beatrice and Princess Eugenie, for example -- can use the designation because they were born into blue blood clan.

Others who married into the family, like Sarah Ferguson and Sophie Rhys-Jones, never received the designation.

As for the late, great Princess Diana -- who's undoubtedly looking down with a smile on this monumental day -- she was never officially designated as Princess Diana, rather the Princess of Wales and Diana, Princess of Wales, after her divorce from Prince Charles.

None of this is set in stone, however, as Queen Elizabeth II could make Kate a Princess of the United Kingdom, which would then entitle her to be called Princess Catherine.

As previously reported, the young lovers wed at Westminster Abbey as an estimated worldwide audience of 2 billion people watched. United at the altar at 11:11 a.m. local time, the duo exchanged vows at 11:20 a.m., after which the hunky royal slipped the ring on his bride's finger.