A publicly known and accepted line of succession is a hallmark of a stable government.
As to the British monarchy, in 2013 the UK finally abolished male-preference primogeniture and also permitted those in line to marry Roman Catholics. The stricture against Roman Catholics actually being monarchs imposed by the 1701 Act of Settlement remains.
As to the US presidency, an early question was whether the number three was to be a member of congress or the secretary of state.
At the time of the debate, the secretary of state was Thomas Jefferson, who later became vice president and still later president. Arguably, he was number three immediately prior to the debate, which throws him up to trivia with the clue “First third, second second, third first (US)”.
Congress eventually opted for its own members, providing as numbers three and four respectively the president pro tempore of the Senate and the speaker of the House of Representatives.
While the line of succession has been changed from time to time, the Secretary of State has always retained a role as the recipient of, and giver of, notice. And so it was on 9 August 1974 when President Nixon gave notice of his resignation to Secretary Kissinger.
