4. Heirs cannot take off together.

Justification: To maintain the succession
Young people, such as Prince George, are an exception.
The line of succession is important to the Royal Family because it determines who will succeed to the throne. This succession was governed by common law, religion, legitimacy, and gender, and the route to the throne has resulted in violent conflicts over the ages.

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However, there are still stringent regulations in place to protect the line, even though relations within the Royal Family have (slightly) calmed down. For example, it is not permitted for two direct heirs to travel by plane together in case the worst happens. Small children are the only exemption, as they must be accompanied by a parent. The Royals would much rather fly in their own private plane than take a British Airways or JetBlue flight.