Although the final orders of the Prime Minister are at his or her discretion, and no fixed options exist, there were four known options: retaliating with nuclear weapons, not retaliating with nuclear weapons, the submarine commander uses his own judgement, or the submarine commander places himself under United States or Australian command if possible. This system of issuing notes containing orders in the event of the head of government's death is said to be unique to the United Kingdom (although the concept of written last orders, particularly of a ship's captain, is a naval tradition), with other nuclear powers using different procedures. The letters are destroyed unopened whenever a Prime Minister leaves office.