Operation London Bridge is the code name for the secret operation that has been meticulously put together for the day when The Queen dies. (There has been a plan in place for what happens when the Queen dies since the 1960s.)
It covers everything from what will happen to her body, to how Australia will be involved.
In 2017, The Guardian published a detailed story explaining in minute detail just what will happen as the United Kingdom and the world contends with the loss of a nearly universally adored figure.
London Bridge is down
The first thing that will happen once the Queen is pronounced dead is her private secretary will contact the Prime Minister. The news will be further relayed on secure lines with the coded message of "London Bridge is down".
The wider world will find out in two ways.
One, a newsflash will go out via the Press Association alerting the world's media.
When the news is made public, TV presenters will don black ties and outfits and pre-prepared obituaries will run. Expect rolling news coverage the world over. Pilots will announce her death to passengers on any planes flying at the time.
Secondly, a footman wearing black will walk out of Buckingham Palace and attach a black-edged notice to the gates.
Leverton & Sons are the royal undertakers and keep a "first call coffin" on hand for royal emergencies, The Guardian has reported. If the Queen dies at one of her homes outside London (Balmoral or Holyrood Palace in Scotland or Sandringham in Norfolk) her body will be transported back to London by the royal train.
Both the upper and lower houses of parliament will sit as soon as possible, flags will be lowered across the nation and the Commonwealth, British workers will be sent home early, and Charles will likely make a live address to the nation that evening.
Any members of the royal family who are travelling or out of the country will return home as soon as possible. It is for this reason they are always required to travel with one black ensemble.
Before the funeral
The Dukes of Norfolk have been in charge of royal funerals since 1672 and as such the 18th Duke of Norfolk, the Earl Marshal, will be in charge. (They have permanent offices at St James's Palace to that end.)
During the 10-day mourning period, the Queen's coffin will lie in state at Westminster Abbey for the public to pay their respects for 23-hours a day, after which she will be given a full state funeral, led by the Archbishop of Canterbury.
In the lead up, the 10 pallbearers will practise carrying her coffin, as members of the royal family are buried in lead-lined coffins.
The day of the funeral
The day of the Queen's funeral will likely become a national holiday and the London Stock Exchange (and most British banks) will close, costing the economy billions of pounds.
On the morning of the funeral, Big Ben's hammer will be covered with leather so its famed tones are muffled and there will be a 41-gun salute from Hyde Park.
Once the service starts, across London's underground, announcements will stop and buses will pull over to the side of the road.
Afterwards, the Queen's body will be driven to Windsor Castle where she will be buried alongside 10 other former monarchs.
The day after the Queen's death, the Ascension Council will meet at St James's Palace and Charles will be proclaimed King at 11am. (The Australian High Commissioner to the United Kingdom will observe the Ascension Council.) That evening, parliament will meet to swear allegiance to the new sovereign.
In the days between then and the funeral, King Charles will visit Scotland, Northern Ireland and Wales.
The Duchess of Cornwall will officially become Queen Camilla. However, earlier this year Clarence House confirmed she will be known as Princess Consort.
The New Prince of Wales
Prince William is likely to inherit the title of the Prince of Wales, although this too will not automatically happen upon the Queen's death.
At a later stage, he will be invested in a lavish ceremony, as Charles was in 1969.
The huge price tag
Both Her Majesty's funeral and Charles' coronation will both be national holidays. It has been estimated that the loss of productivity for both days will cost the UK somewhere between A$2.1 and $10.8 billion.
The role of head of the Commonwealth is not hereditary. However, in 2018 at a Commonwealth Heads of Government "retreat" held at Windsor Castle, it was decided that Charles would be the next head.