The 67-year union between the Queen and her husband, Prince Philip, is at the heart of a family which has endured through decades of change. Their marriage is also the key to the sovereign's record-breaking reign, which on 9 September will officially be longer than Queen Victoria's.
Elizabeth II first met the man she refers to as her "strength and stay" at the tender age of 13 when she visited Dartmouth Naval College. Philip, a promising young cadet and a member of the Greek royal family, had been assigned to show her around.
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Despite their shared royal pedigree, their respective childhoods couldn’t have been more different.
While Elizabeth grew up in a stable and privileged environment, Philip's parents were divorced and his family were exiled from Greece.
His father lived in the south of France, and his mother spent time being treated in a psychiatric clinic.
In visitors' books, he described himself as "of no fixed abode" and when asked by a journalist what language he spoke at home he bluntly replied: "What do you mean, at home?"
But something about the young officer tugged at the heartstrings of his future wife. After their initial introduction, the shy young princess remained devoted to the nomadic 18-year-old, regularly exchanging love letters with him, despite the insistence of her father, King George VI, that she "meet more men".
In one romantic exchange, the smitten war hero wrote: "To have been spared in the war and seen victory, to have been given the chance to rest and to re-adjust myself, to have fallen in love completely and unreservedly, makes all one’s personal and even the world’s troubles seem small and petty".
In 1946, the couple secretly became engaged – a controversial decision since Elizabeth's family was opposed to her marrying so young.
But in April 1947, the King finally softened and gave them permission to wed.
This prompted Philip to renounce his Greek citizenship and take the surname Mountbatten from his British maternal grandparents. His mother Princess Alice was the great-granddaughter of Queen Victoria.
On November 20, 1947, the young lovers' wedding day dawned. Some 2,000 guest witnessed the ceremony, held in Westminster Abbey and broadcast to 200 million listeners around the world.
With Britain still feeling the pinch following the war, extra ration cards were needed to purchase material for the bride's flowing ivory satin gown, which was adorned with around 10,000 pearls imported from America.
On his marriage the groom was styled His Royal Highness The Duke of Edinburgh.
Since then he has taken his place one step behind his wife on all official occasions and the countless overseas tours they made together.
Over the years, the couple's union has been cemented by a devotion to duty and shared passions, such as their love of equestrian sports.
He makes her laugh and is one of the few people who remembers her simply as 'Lilibet'.
But, despite being an exemplar for a lasting marriage, they have their ups and downs like everyone else.
In a rare and fascinating insight, royal biographer, Robert Hardman described a fraught moment during a tour of Australia in 1954 when the Queen hurled a tennis racquet at her husband's head in a fit of rage.
A film crew following the couple on their tour caught the domestic on camera.
And Philip is the only one permitted to be abrupt with her. Once, when the Queen irritated him by sucking in her breath at his fast driving, he told her: "If you do that once more I shall put you out of the car."
But if pressed by anyone who had the indiscretion to ask about their rows, the Duke would probably just shrug his shoulders and repeat as he once said: "We are a family. What do you expect?"