For a man who once listed the pros and cons of getting married on a whiteboard before he took the plunge, Bill Gates was probably never going to be the most conventional husband.

And that was before the Microsoft founder was named the world's richest person and those pros and cons started to become just a little more complicated.

History doesn't record whether he factored divorce into his calculations but 27 years later, as he and wife Melinda announce their marriage is 'irretrievably broken', questions remain over what precisely drove them apart and what will happen to a family fortune valued by the Bloomberg Billionaires Index at $146 billion.

The couple are not believed to have signed a pre-nuptial agreement but have instead worked out an undisclosed separation agreement.

Bill and Melinda have pledged to give half their fortune to charity by the time they die and their $50 billion foundation is the world's biggest philanthropic entity, giving away about $5 billion a year to public health and development initiatives.

The couple are also the largest owners of farmland in the U.S., not to mention a string of fabulous homes, including a $125 million, 66,000-sq ft Washington state mansion. He remains the world's fourth richest person.

Even if Gates, 65, is only the latest in a succession of billionaires who have been unable to make their marriages last, the announcement still sent shock waves through the worlds of technology and philanthropy where the couple had seemed a united presence.

'After a great deal of thought and a lot of work on our relationship, we have made the decision to end our marriage,' they said in a brief statement. 'We have raised three incredible children and built a foundation that works all over the world to enable all people to lead healthy, productive lives . . .

'We no longer believe we can grow together as a couple in this next phase of our lives.'

Their eldest child, Jennifer, fleshed out that bland, corporate-sounding announcement, writing online that the family had been going through a 'challenging stretch of time'.

The couple have reportedly struggled to keep their relationship going in recent years and there have been claims it has almost collapsed several times.

Mr Gates decided to step down last year from the board of Microsoft and another large company, Berkshire Hathaway, in part so he could spend more time with his family.

Melinda, 56, has dropped the occasional hint that her workaholic husband was not the easiest man to live with. She once described their marriage as 'incredibly hard' as Gates can work 16-hour days and be detached from family life.

'When he was having trouble making the decision about getting married, he was incredibly clear that it was not about me, it was about: 'Can I get the balance right between work and family life?' ' she said in 2019.

'Believe me, I can remember some days that were so incredibly hard in our marriage where you thought, 'Can I do this?' '

Two years ago, she said patience had been key, although she said workaholic Gates was fairly easy to cope with most of the time. Nevertheless, she once recalled being furious with him because he was reading a book about Winston Churchill instead of helping her pack the car and get the children ready.

Despite his reputation as a socially awkward nerd, Gates had a torrid seven-month affair with another attractive Microsoft executive, Stefanie Reichel, in 1992 — in between meeting and marrying Melinda.

Another girlfriend until 1987 was venture capitalist Ann Winblad, whose approval of Melinda he actually sought before he agreed to marry her. Bizarrely, the Gateses had an agreement lasting decades that he leave her each year and spend a long weekend beach holiday with Ms Winblad.

However, his colourful romantic life, as well as Gates's history as a ruthless businessman who eliminated rivals and allegedly plotted against Microsoft co-founder Paul Allen after the latter was diagnosed with cancer, tends to be forgotten as he has basked in his image as devoted family man and crusader for global health.

Insiders credit his wife for putting him on the 'good deeds' path. They met shortly after she joined Microsoft in 1987 as a product manager.

She was on a trip to New York for the Seattle-based company and found herself next to him at a dinner. Melinda said she could tell he was interested in her but took his time making a move.

He finally ambushed her one Saturday afternoon in the Microsoft car park and asked her out on a Friday two weeks ahead. 'I laughed and said, 'That's not spontaneous enough for me. Ask me out closer to the date.' And I gave him my number,' she recalled.

'Two hours later, he called me at home and invited me out for that evening. 'Is this spontaneous enough for you?' he asked.'

She says they discovered they had much in common. They both loved puzzles and were ultra competitive. 'So we had puzzle contests and played math games,' says Melinda who believes Bill became 'intrigued' by her when she beat him at the murder whodunnit board game Cluedo.

He was impressed to discover she had read his favourite novel, The Great Gatsby, twice. 'Maybe that's when he knew he'd met his match. His romantic match, he would say. I knew I'd met my match when I saw his music collection — lots of Frank Sinatra and Dionne Warwick,' she said.

They initially had a casual relationship. 'She had other boyfriends and I had Microsoft,' said Gates. He said he was 'surprised' that, a year into his relationship, they both said: 'I love you.'

In fact, Gates wasn't only seeing Microsoft. Stefanie Reichel, a 25-year-old Microsoft sales manager based in Germany, met him at a business meeting in Monte Carlo and he started bombarding her with embarrassingly romantic emails.

During their seven-month affair, they stayed in luxury hotels in London, at the gatekeeper's cottage of his Seattle estate and even visited Amsterdam's red light district.

Ms Reichel, who said she ended the affair despite his pleas for her to reconsider, later insisted she found him repulsive but was worried for her job if she resisted his advances.

Melinda Gates admits Bill, who dropped out of Harvard to set up Microsoft, eventually had to decide whether he wanted to get married or end their relationship.

She revealed that she once walked into his bedroom and he was making a list on a whiteboard which 'had the pros and cons of getting married'. Neither of them has ever revealed what he had written, although Gates said his wonderfully unromantic approach simply showed that 'I took the idea of marriage very seriously'.

His own parents — Bill Snr, a lawyer, and Mary, a businesswoman — both had careers, Melinda told a 2019 documentary. Gates 'wanted to be married but he didn't know whether he could actually commit to it and [run] Microsoft,' she added.

They finally wed in Hawaii on New Year's Day 1994. She was 29 and he was 38, Gates later hailing his marriage as a 'truly equal' partnership. Melinda, however, acknowledged that the fact that she also worked for Microsoft had been 'a huge part of our life together', so much so that she considered delaying telling him she was pregnant until they had taken a holiday in China because she wanted Gates to relax.

However, he was shocked, she said, when — before Jennifer was born in 1996 — she said she wanted to be a stay-at-home mother. 'He was stunned,' she recalled, saying he told her: 'What do you mean, you're not going back?'

Melinda, who has described herself as an 'ardent feminist', said she felt 'very alone in our marriage' after giving birth to Jennifer because Gates was spending so much time at work. 'He was beyond busy; everyone wanted him, and I was thinking, 'OK, maybe he wanted to have kids in theory, but not in reality.' '

They persevered at trying to work out more equal roles at home. Jennifer, is now 25 and engaged to an Egyptian showjumper while Rory is 21 and Phoebe is 18.

Gates would drive Jennifer to school and did the washing up at night, even if billionaire investor Warren Buffett was coming round to play bridge (along with tennis and golf, one of Bill's off-duty passions).

Melinda says she has been obsessed with her own family being as 'normal' as possible and the children not becoming entitled. Although they have a cook, the siblings had to tidy their rooms, walk the dogs and lay the table. The three have been told they will inherit 'just' $10 million as their parents give the rest away. Gates says he is doing them a 'favour'.

The children also had to live with a ban on Apple products at home, a legacy of Gates' rivalry with Apple guru Steve Jobs.

In other areas, they've been less successful at being normal. While she and the understated Gates prefer comfortable chinos to designer clothes (he also wears a $10 watch and has a penchant for fast-food burgers), they have a $125 million home outside Seattle, three other homes, a private jet and a multi-million-dollarart collection.

The couple have adjoining offices at their foundation's Seattle HQ and meditate together every day.

Gates has praised his wife for softening his nerdy edges. 'She's a lot like me in that she is optimistic and she is interested in science,' he said. 'She is better with people than I am. She's a tiny bit less hard core about knowing, you know, immunology, than I am.'

In 2014, Gates stepped down as chairman of Microsoft to focus on the Bill and Melinda Gates Foundation, the philanthropic giant they set up in 2000. In 2010, the couple created The Giving Pledge with Warren Buffett and encouraged other billionaires to follow them and give away half their fortune.

Critics say the couple, through their foundation, have too much power over decisions on global public health that should better be left to governments. Some charities have talked of so-called 'Bill Chill' — the threat they face of offending the Gateses and missing out on their money by criticising their views.

When Amazon chief Jeff Bezos divorced his wife, MacKenzie Scott, in 2019, giving her shares in the company worth $36 billion, she gave billions away.

Many believe Melinda will follow suit — much of Gates' Microsoft money has still not been given away and she may rectify that, leaving even his philanthropy in the shade.

After so many years putting up with the brilliant but difficult tycoon who created the company, surely few would deny her that privilege.

FROM PRIVATE JETS TO A DA VINCI, THE RICHES THEY'LL SPLIT

BUSY ECO-WARRIOR'S FLEET OF AIRCRAFT

Gates, who plans out his days in five-minute blocks, justifies owning several private jets to achieve the maximum efficiency. The avowed eco-warrior, who until 1997 flew everywhere in economy class, owns a fleet of aircraft which reportedly includes two $65 million Gulfstream G650ERs (one pictured here), two Bombardier Challenger 350s and a Cessna Seaplane — as well as a collection of helicopters.

FIVE HOMES IN THE LAP OF LUXURY...

The Gates family's main home is known as Xanadu 2.0. A 66,000 sq ft mega-mansion overlooking Lake Washington near Seattle, it is estimated to be worth $125 million.

Sprawling across five acres, it has seven bedrooms, 18 bathrooms and a 20-room main living area, as well as a 30-car garage.

It boasts a trampoline room, an art-deco screening room, an outdoor-indoor pool with an underwater sound system and a multi-room library filled with rare documents and artifacts.

The 1,000 sq ft dining room seats 24 but is dwarfed by a 2,300 sq ft reception room that fits 200.

The huge library includes two secret pivoting bookcases, one of which contains a bar.

The estate features a stream stocked with salmon and trout, and a beach with sand said to have been imported from the Caribbean.

'I wanted craftsmanship but nothing ostentatious,' said Gates of the property, apparently without irony.

The couple also own a $59 million ranch in Florida and Irma Lake Lodge and a $9 million, 492-acre Wyoming ranch, which was originally settled by Buffalo Bill.

The family has two more homes in California, including another ranch which has its own racetrack and a $43 million ocean-front mansion in San Diego.

EVEN HIS ELECTRIC CAR IS A PORSCHE...

Gates loves cars, and his collection includes a Porsche 930 Turbo, a Jaguar XJ6 and a Ferrari 348. His most valuable car is believed to be an extremely rare Porsche 959 — only several hundred of which were ever made — and sell at auction for up to $2 million. Even his electric car is a Porsche — the sporty Taycan, like the one above — a brand to which he has been loyal since he bought a 911 in 1979.

$31m FOR LEONARDO'S SCIENTIFIC NOTEBOOK

Xanadu 2.0 is the main home for the multi-million-dollar Gates art and valuable book collection.

Bill has four rare copies of his favourite novel, The Great Gatsby, on the library shelves but the most valuable tome is a scientific notebook written by Leonardo da Vinci.

He bought Leonardo's Codex Leicester (right) — so named because it was once owned by the Earl of Leicester — for $31 million at auction.

Gates's art collection includes Winslow Homer's seascape oil painting, Lost On The Grand Banks, which he bought for $36 million in 1998.

He also owns George Bellows's Polo Crowd, for which he paid $28 million the following year, Childe Hassam's Room Of Flowers, which he snapped up for $20 million, and William Merritt Chase's The Nursery (left), valued at $10 million.