When an outgoing president hands the keys to the White House to the incoming one, another discrete handover takes place: the systems and codes for the US leader to launch a nuclear strike.
On Wednesday Donald Trump's huffy refusal to attend successor Joe Biden's inauguration created an unprecedented challenge.
Trump travelled to Florida three hours before Biden was sworn in and, as he was still president, he took with him the nuclear "football," the bag containing the procedures and equipment for a launch.
He also would have had in his pocket the "biscuit" -- a card of authentication codes for embarking on a nuclear launch that only the president can use.
Biden, though, needed the launch codes and communications at his side from the moment he became president and commander in chief, at noon in Washington.
So for the first time in a transfer of presidential power, there was a second active "football" -- actually a hefty black leather briefcase always carried by the president's military aide -- for the occasion.
At noon, just as he took the oath of office, Biden had a military officer standing nearby with the duplicate nuclear briefcase, and he received his own biscuit. Trump's meanwhile expired, like an outdated credit card.
While it was a new test of maintaining unbroken presidential control over conducting nuclear war, it wasn't actually too difficult.
There are always three footballs around: one with the president, another with the vice president in case he has to assume power, and third backup.
In the final days of Trump's administration, and especially after the January 6 attack on the Congress by supporters of Trump, some concern was expressed about him retaining the power to order a nuclear attack.