In the British royal line of succession, the heir apparent is the eldest child of the monarch; currently, Queen Elizabeth II's heir apparent is her eldest son, Charles, Prince of Wales. Next in line to the throne is the heir apparent's eldest child -- Prince William, Duke of Cambridge. When a direct heir has children, those children follow the heir in the line of succession, preceding the siblings of the direct heirs. Thus, Prince William's children precede his brother, Prince Harry, in the royal line of succession. According to British royal protocol, the heir to the throne and his eldest child may not travel on the same plane, to protect the royal lineage in case of a crash.
The British Royal Family almost always follows this travel protocol. However, there have been several occasions when the protocol was broken. For example, Queen Elizabeth II gave special permission for Prince Charles, Prince William, and Prince Harry to travel on the same plane after the death of the Queen Mother in 2002.
More about the British Royal Family:
- Elizabeth Bowes-Lyon, the Queen Mother (mother of Queen Elizabeth II), died in 2002, at the age of 101.
- Prince Charles met his current wife, Camilla Parker Bowles, at a polo match when he was 22 years old.
- Prince William's full title is "His Royal Highness Prince William Arthur Philip Louis, Duke of Cambridge, Earl of Strathearn, Baron Carrickfergus, Royal Knight Companion of the Most Noble Order of the Garter."