The same interview pointed out that while the Prime Minister would have the constitutional authority to fire the Chief of the Defence Staff, he could not appoint a replacement as the position is appointed by the monarch. The programme also addressed the workings of the system; detailing that two persons are required to authenticate each stage of the process before launching, with the submarine captain only able to access the firing trigger after two safes have been opened with keys held by the ship's executive and weapons engineering officers. It was explained that all Prime Ministers issue hand-written orders, termed the letters of last resort, seen by their eyes only, sealed and stored within the safes of each of the four Royal Navy Vanguard class submarines. These notes instruct the submarine commander of what action to take in the event of the United Kingdom being attacked with nuclear weapons that destroy Her Majesty's Government in the United Kingdom and/or the chain of command.