Sometimes it seems unthinkable: For every step of Queen Elizabeth's 69-year reign, Prince Philip has been here by her side, her constant, as she once touchingly put it, "strength and stay".
Despite a number of hospitalisations in recent years, Philip has defied the odds and will turn 100 years old in June this year like some sort of Lazarus in a Savile Row suit.
Prince Philip has just spent his seventh night in hospital, with Buckingham Palace releasing a statement on his condition.
When the tragic day of his death does come, the details of what will happen, down to the most minute point, have long been put in place.
Operation Forth Bridge will be triggered
As macabre as it might sound, there have long been specific, exhaustive plans in place for the deaths of senior members of the royal family, all of which have codenames. For example, the Queen's is called Operation London Bridge and Prince Charles' is Operation Menai Bridge.
When Diana, Princess of Wales was killed in Paris in 1997 there was no blueprint in place for her death and courtiers instead copied the blueprint they already had for the Queen Mother's death — Operation Tay Bridge.
Philip's plan is called Operation Forth Bridge.
How the public will be told
Well, that depends on what time of day or night he dies.
Obviously, the Queen will be the first person told, if she is not by his bedside, followed by family members. Next comes the Lord Chamberlain who is the head of the royal household, a position currently held by the ex-MI5 chief Andrew Parker. He in turn will tell the British Prime Minister. It is only at this stage that the outside world will be told the sad news.
The BBC, as the national broadcaster, will most likely be the first to report the news. If it happens overnight, the announcement is expected to be made at 8am London time, which is currently 9pm NZT.
How the media will react
The press reaction will be swift and largely predictable. For years — decades even — magazines, TV news shows and newspapers have been preparing for when both the Queen and her husband sadly breathe their last. Obituaries will be immediately published, ready-made documentaries will air and commentators will clog the airwaves 24/7.
Expect to see TV anchors and all reporters wearing black.
Flags will be lowered - but not at Buckingham Palace
Across the UK and Commonwealth, flags will be lowered to half-mast as a sign of respect.
Crucially though, the Royal Standard Flag, which is flown at royal residences when Her Majesty is present, will remain flying. This particular flag symbolises the continuity of the monarchy and is only ever lowered when a sovereign passes away.
Elsewhere, in the British Houses of Parliament, the ceremonial "mace" will be covered by a black cloth and Members of Parliament will don either black ties or black armbands.
No laws will be passed for eight days
The Queen will begin an official mourning period which is expected to last eight days. Some British government affairs might be put on hold for this period, as during this time no laws will be given the Royal Assent.
The Royal Family will assemble
Now, this is dependent on what Covid restrictions are like when this day comes. Under current rules, they would not be allowed to leave their respective homes (Highgrove in Gloucestershire for Prince Charles and Camilla and Anmer Hall in Norfolk for Prince William and Kate) to visit the Queen.
If Philip's death happens once things have more or less returned to normal then it is likely the Queen's children, grandchildren and great-children would head to Windsor to support Her Majesty during this sad time.
Black suits and no social engagements
All members of the royal family travel at all times with a black outfit on hand, just in case. (This rule came into effect after the Queen's father King George VI died in 1952. At the time, she and Philip were in Africa and immediately flew home to London, but had not packed any mourning clothes. When their plane landed back in the UK, suitably sombre clothes had to be brought onto their aircraft for them to change into before they emerged to face the press.)
From the minute the news of the Duke's death breaks, any time we see a member of the house of Windsor, they will be in black and — for the men — potentially wearing black armbands too.
The family will cancel all social events although there is a question mark over whether they would go ahead with some prearranged official engagements.
No state funeral
As the Queen's official consort, Philip is entitled to a state funeral, however in his classic no-nonsense style, he wants a far simpler and more personal service.
Rather than lying in state in Westminster Hall as he would be entitled to, it has been reported that his body will lie at St James's Palace (which is also where Diana's body lay before her funeral). Covid restrictions notwithstanding, it is unlikely there will be any sort of public viewing.
Military pomp and no cameras
Philip's funeral service, rather than Westminster Abbey, will be held at St George's Chapel at Windsor (yes, where Harry and Meghan, the Duke and Duchess of Sussex got married).
Even before the pandemic, the plan was for the event to be quite low-key, with only family members and some heads of state attending. It is believed that Philip, who served in the British navy for 13 years including during World War II, has chosen what is essentially a military funeral.
It is also unlikely, given the relatively personal commemoration of his life, that TV cameras would be allowed inside the chapel, as they are at, say, royal weddings.
He will be buried near Harry and Meghan
Afterwards, it is believed he will be buried in Frogmore Gardens, which is the final resting place for Queen Victoria and Prince Albert. Also on the Frogmore Estate is the official British residence of the Sussexes — and source of much controversy — Frogmore Cottage.
The Queen will go back to work
It is unthinkable what it would be like for Her Majesty to have to return to her daily life without her partner of 73 years. (Their 74th wedding anniversary would be in November 2021.) However, have no doubt: For the Queen, duty and service to her country have been her lodestars since she ascended to the throne in 1952.
While Queen Victoria, who after the sudden loss of her beloved "Bertie" disappeared from public life and spent years in seclusion, our current Queen will return to carrying out her duties and events with the determination and commitment that have been the hallmarks of her reign.