Offenders know this. Kate and William learned this the hard way during a helicopter trip.
The Duke and Duchess of Cambridge had the imprudence to climb together, accompanied by their three children, in the device when it is strictly prohibited.
The protocol indeed requires that the members of the royal family do not travel together for security reasons. This is especially true for those who are called to wear the crown.
Back from their trip, William and Kate will be asked to explain themselves to the Queen who summoned them.
