interregnum

Every time a pope dies, there's an interregnum period before a new one is elected by the cardinals. In most democratic systems, however, the law specifies who should take office when a president or prime minister dies unexpectedly, and since the power usually passes automatically, there's no true interregnum. The question of succession—that is, of who should take over when a country's leader dies—has often presented huge problems for countries that lacked a constitution, and in monarchies it hasn't always been clear who should become king or queen when a monarch dies. The interregnum following the death of Edward VI in 1553, for instance, was briefly suspended when Lady Jane Grey was installed as Queen; nine days later she was replaced by Mary Tudor, who sent her straight to the Tower of London.