The death of a monarch is something the UK hasn't had to deal with in more than six decades.
Just what will happen when 90-year-old Queen Elizabeth II dies has now been revealed.
According to UK paper The Guardian, the first anyone outside of her immediate circle will hear is a codeword: 'London Bridge is down.'
"Her eyes will be closed and Charles will be king. His siblings will kiss his hands."
Sir Christopher Geidt, the Queen's private secretary who has been knighted twice, is the man tasked with telling the wider world.
According to The Guardian, the first thing he'll do is call the UK Prime Minister - currently Theresa May - and pass on the codeword. If she's asleep, she'll be woken.
Telling the other 15 countries where Elizabeth is head of state will be up to the UK government's Foreign Office.
The press will find out soon after, and the BBC won't get special treatment - no British media will, with the broadcast going out to everyone at the same time.
As that happens, a notice will be pinned to the gates of Buckingham Palace announcing the Queens' passing, and the palace website will transform into a single page bearing the same text.
Radio stations across the UK have 'obit lights' to warn DJs news of a "national catastrophe" is about to break. They're urged to play "inoffensive" music while they wait for the news to arrive.
Male TV newsreaders will don black suits and ties, interrupting whatever else is going to air at the time.
"The royal standard will appear on the screen. The national anthem will play," reports The Guardian. "You will remember where you were."
Britons at work will likely be asked to go home. Both houses of the British parliament will be recalled.
If she dies overseas, a jet will be dispatched from western London, coffin on board, to retrieve her.
It's most likely when her time comes she'll be at Balmoral, where she spends three months a year. If so, she'll lie in rest at a small palace in Edinburgh, before being carried to a royal train at Waverley, before heading south to Buckingham Palace.
Most of the planning was done in the 1960s, with refinements made over the years. But just what happens in everyday life remains a mystery, it's been so long since a monarch died. Sports matches were called off following the death of George VI in 1952, but whether that happens again could be up to the newly crowned King.
Flags will only remain at half-mast for a day, going back up after a single night. At 11am the following morning, Charles will be crowned King, and Camilla, his legal Queen - but whether she takes on that formal title remains to be seen.
Elizabeth's funeral will be held nine days after her death. At the moment her coffin reaches the doors of Westminster Abbey, life in the UK will come to a standstill.
"Train stations will cease announcements," reports The Guardian. "Buses will stop and drivers will get out at the side of the road."
Read the full sequence of events on The Guardian's website.