A good sense of humour features high on dating websites as a desirable quality ("GSOH"), but with the 70-year marriage of the Queen and the Duke of Edinburgh, it is more a case of SSOH - a shared sense of humour.
Those who know the couple best say it has been their enduring ability to laugh at the same things that has sustained their partnership over decades of intense scrutiny in the public spotlight, reports the Telegraph.
"You have to list companionship, friendship and shared experience as having seen them through to this remarkable anniversary," says Jennie Bond, former BBC royal correspondent, "as well as a very deep love between them. But a big part of what has kept them going over so many years is the fact they can share a joke together."
From her 14 years of covering royal tours around the globe, Bond recalls one episode in particular to illustrate her point. In October 2002, the royal couple were in Winnipeg in Canada.
"It was absolutely freezing and they had just sat outdoors through what was officially labelled a 'cultural display' of song and dance that went on and on and on. Then they were deposited onto a barge, which promptly broke down in the middle of the Red River and had to be rescued. When the Queen and the Duke finally got back to land, everyone was expecting them to look traumatised or frostbitten, but instead you couldn't mistake the amusement on their faces at the whole episode."
Because they spend some much of their time going from one stage-managed event to another, planned down to the nth degree, Bond suggests it "tickles" them both when things go wrong. "It is how they have survived being on show for so long."
In private, the Queen is said to be a fine mimic, particularly skilled at pulling funny faces, and capable of reducing a whole dinner table to fits of laughter. In public, though, she is much more reserved. On occasion, the two sides of her have overlapped, usually when the Duke of Edinburgh is around.
An early picture of the couple, taken in Wales in May 1963 while watching soldiers at a military camp, has the Queen almost doubling up with laughter at something her husband, standing behind her, has said. And in April 2003, she was inspecting the Queen's Company of the Grenadier Guards at Windsor Castle.
As their Colonel, Prince Philip was in attendance in full ceremonial uniform, including a bearskin. The cameras caught the moment as she walked past him, his face still but a glint in his eyes, which sets her off giggling.
"That was all to do with presence of a bee," explains Hugo Vickers, biographer of both the Queen Mother and Princess Alice of Greece, the Duke's mother. "It's a good illustration of how Prince Philip has always seen it as his role to keep his wife's spirits up during public events, and keep her feeling jolly about things. It is what makes them such a good double act."
The Duke even has a pet name for her. The rest of her close family may call her Lilibet, but he apparently refers to our sovereign as "Cabbage". This was revealed by screenwriter, Peter Morgan, at the launch of his 2006 film, The Queen.
He said he had got it on good authority from Buckingham Palace staff and therefore had included a scene where James Cromwell, playing the Duke, orders Dame Helen Mirren's Queen "Move over, cabbage" as they get into their double bed.
Over their 70 years together, there has been plenty of evidence of a private intimacy, even when in a crowd, that is created by knowing what the other is thinking - and finding it funny.
When the Queen sat down to record her first-ever televised Christmas message in 1957, she was racked with nerves and froze in front of the cameras. The news was quickly conveyed to Prince Philip, who immediately sent a message back to the director. "Tell her to remember the wailing and gnashing of teeth".
To everyone else in the room, his words meant nothing when read out to Her Majesty, but it made her smile. The ice was broken and the recording could go ahead.
The private reference was to what courtiers have described as the "screams of laughter" from the Queen when her husband would run up and down corridors brandishing a pair of false teeth to amuse her and their young children.
That shared schoolboyish humour - later said to have been inherited by their son, the Duke of York, who would put a whoopee cushion on the Queen Mother's chair - was very much in evidence on some of the couple's early overseas tours.
In 1951, on another tour of Canada, which entailed long rail journeys from one event to another, the Duke is reported to have sent his valet to a joke shop to buy an imitation tin of nuts. He then placed it on the table between them in the royal train. When the Queen opened it to reach in for a snack, a toy snake popped out - to her delight.
His humour can be slapstick, and it can be dry, especially when he wants to apply balm to stressful situations. In 1964, the Queen chose to have a home birth with her last child, Prince Edward. A guest bathroom at Buckingham Palace was therefore commandeered for the occasions. Prince Philip was present for the birth and, as the labour went on for longer than expected, decided to lighten the atmosphere by remarking to his wife: "It's a solemn thought that only a week ago, General de Gaulle was having a bath in this room..."
The Duke's asides are, of course, not always to everyone else's taste. He enjoys a reputation for putting his foot in it. At a reception in 2010 to welcome Pope Benedict to Edinburgh, he caused outrage when he asked the then Scottish Tory leader, Annabel Goldie, if she had any tartan knickers.
"That is just the Duke trying to break the ice," says Bond. "Again, it is that reflex of his to use humour to protect the Queen on such occasions. She can be quite standoffish, so he plunges in with people with this very dry sense of humour. Occasionally, though, he not only breaks the ice, but falls through it."
And, adds Vickers, the Queen herself is not above uttering the occasional dry remark in public. "She has this calm, level gaze, and her enormous skill is her restraint, but if something is said to her at a reception that amuses her, she might reply: 'How very unusual...'"
With her staff, she is said to be adept at teasing while keeping a straight face. Philip Moore, later ennobled and her personal private secretary from 1977 to 1985, described how Her Majesty once asked to look over Buckingham Palace's household accounts to make some economies. When he asked her what she had decided, she replied, as if serious, that she thought they might cut back on ink by stopping dotting the i's and crossing the t's in correspondence.
And there is even a story - never confirmed - in some royal biographies that the Queen is not above slapstick herself. She is said to have installed a speaker in one of the bathrooms in Buckingham Palace's private quarters. When someone sat on the toilet, it played a recording of her voice saying: "Do you mind! I'm working down here."
On their annual summer holiday to Balmoral, the royal couple can relax and allow their public and private faces to merge. At their Aberdeenshire castle, with family and distinguished guests, they happily indulge their apparently insatiable appetite for games of charades and elaborate picnics, where they make a virtue out of taking no account of the weather and are amused if visiting presidents or prime ministers look uncomfortable sitting out eating in the rain.
Outside the royal estate, too, that spirit has been captured by photographers. In 2007, at the Braemar Highland Games during their summer break, in an unusually intimate picture, the Queen reaches out her arm towards Prince Philip as if to nudge him into sharing whatever joke it was that amused her. On another occasion, at the same gathering, the two of them are sitting snugly under a tartan rug, with their eldest son, Prince Charles, alongside them, chuckling away.
These are the rare glimpses that reveal an often hidden side of their marriage. After 70 years together, the Queen and Prince Philip have settled into the kind of cosy companionship that comes with age - and to which most of us can only aspire.