At the moment the Queen died, the throne passed immediately and without ceremony to the heir, Charles, the former Prince of Wales.
But there are a number of practical – and traditional – steps which he must go through to be crowned King.
He will be known as King Charles III.
That was the first decision of the new king’s reign. He could have chosen from any of his four names – Charles Philip Arthur George.
He is not the only one who faces a change of title. Prince William and his wife Catherine are now titled Duke and Duchess of Cornwall and Cambridge, and the king has conferred on them the title of Prince and Princess of Wales.
There is also a new title for Charles’ wife, Camilla, who becomes the Queen Consort – consort is the term used for the spouse of the monarch.
Charles will be officially proclaimed King on Saturday. This will happen at St James’s Palace in London, in front of a ceremonial body known as the Accession Council.
This is made up of members of the Privy Council – a group of senior MPs, past and present, and peers – as well as some senior civil servants, Commonwealth high commissioners, and the Lord Mayor of London.
More than 700 people are entitled in theory to attend, but given the short notice, the actual number is likely to be far fewer. At the last Accession Council in 1952, about 200 attended.
At the meeting, the death of Queen Elizabeth will be announced by the Lord President of the Privy Council (currently Penny Mordaunt MP), and a proclamation will be read aloud.
The wording of the proclamation can change, but it has traditionally been a series of prayers and pledges, commending the previous monarch and pledging support for the new one.
This proclamation is then signed by a number of senior figures including the prime minister, the Archbishop of Canterbury, and the Lord Chancellor.
As with all these ceremonies, there will be attention paid to what might have been altered, added or updated, as a sign of a new era.