Details of the British government's plan for the Queen's death — Operation London Bridge — have been well-reported over the years.
But now that the Queen has died in Scotland, a second, lesser-known plan called Operation Unicorn has to unfold first.
The Queen's body will be carried in procession through Edinburgh's medieval heart before a slow train journey past huge crowds of mourners on the way to London.
Here's how Scotland's Operation Unicorn is unfolding.
What is Operation Unicorn?
Unicorn lays out how the Queen's body will be taken from Balmoral to London.
The plan was leaked in 2019 when minutes from a Scottish parliament leadership group meeting were posted on a forum for senior parliamentary staff.
With the Queen typically spending three months a year at her favourite residence, Balmoral, a plan was clearly needed if she died there.
A number of other plans also existed, including Operation Overstudy, which would have been used if the Queen died while overseas.
Where is the Queen’s body now?
The Queen died at Balmoral in north-east Scotland on Thursday local time. Her body remains at the castle at the time of writing.
It is expected her body will be transported from Balmoral to the nearby city of Aberdeen on Friday morning local time.
It will then travel down Scotland's east coast to the capital, Edinburgh.
Mourners are expected to line the route and kilted soldiers will form guards of honour at stations along the way.
In Edinburgh, the Queen's coffin will lie at the official royal residence, a medieval palace called Holyroodhouse, for a day.
The next day her body will travel in procession up Edinburgh's historic Royal Mile to the ancient St Giles' Cathedral, near Edinburgh Castle.
Holyroodhouse and St Giles are expected to be the main sites for people to pay their respects in Scotland.
Operation London Bridge unfolds
Following the ceremonies in Scotland's capital, the next day the Queen's body will likely be put on the Royal Train for the journey south to London.
The train will travel from Edinburgh's Waverley station to St Pancras in London, but it could also be flown south.
Once the Queen crosses the Scottish border, Operation Unicorn ends and Operation London Bridge commences.
Members of the armed forces and emergency services will form an honour guard on every platform of the route.
Large crowds are expected to gather along the way, and the train is expected to travel slowly down the East Coast main line to allow people to pay their respects and share tributes.
The Guardian has reported that level crossings, bridges and embankments are expected to be so overwhelmed with floral tributes that railway officials plan to send a sweeper train behind to clear and tidy up the tracks behind it.
It is expected there will be a high police presence in London as her body is taken from the train, while the RAF will reportedly provide air cover along the route.
Buckingham Palace, then Westminster
In London the Queen's body will lie in Buckingham Palace's throne room before being placed onto a gun carriage for the journey to Westminster Hall, where an official period of lying in state for four days will begin.
On the day of the funeral, the Queen's coffin will be carried to Westminster Abbey as the chimes of Big Ben ring out across the city and the country grinds to a halt.
It will be one last chance for millions of people to say goodbye to the only monarch most of them have ever known.
