Forty years ago, a London publisher was working on a groundbreaking sex manual - a "gourmet guide" to sexual pleasure, with copious and detailed illustrations. But how could this be done tastefully and legally?
Think of The Joy of Sex and chances are your mind will drift to an image of a man with a bushy beard and a woman with hairy armpits.
It's not a photograph, but the nearest thing to it in pen and ink.
In early 1970s Britain, photographs would have been too risque. But hand-drawn illustrations based on photographs? Maybe society was ready for that.
"We were a bit nervous when we took this on," remembers one of the book's illustrators, Chris Foss.
"The publisher had to write a contract which confirmed that they would pay our defence if some old fart decided to make an issue out of it."
In the summer of 1971, Britain had been gripped by the Oz trial, in which the editors of a satirical magazine were found guilty of obscenity for publishing a sexualised parody of the children's comic character Rupert Bear. (The judge was famously called a "boring old fart" in court by a defence witness, the comedian Marty Feldman.)
It was also only 11 years since Penguin Books had faced an obscenity charge for publishing DH Lawrence's Lady Chatterley's Lover - the full text, complete with four-letter words and descriptions of sex between the lady and the gamekeeper.
But Penguin had been found not guilty thanks to the literary merit of the work, so Joy of Sex art director Peter Kindersley calculated that the quality of the art work would shield the publishers, Mitchell Beazley, from prosecution.
The images were graphic - they showed genitals and countless sex positions - but they were also artistic, and tasteful.
For good measure, he also added a number of historical images from India and Japan.
"There was concern about the explicitness of the pictures and therefore we thought as a foil we would put in some of these ancient pictures," says Mr Kindersley.
"In a way we were relating ourselves to the past… We wanted to make the book feel as though it was related to a great tradition of explicit pictures."
But before the artists could start work the team had to find models to pose for them.
Plan A, explains Mr Kindersley, was to use models from London's Soho district - the hotbed of the capital's sex industry.
"We found all these people who started posing, but halfway through the pose they would ask for an extra £100 ($160) - it was just complete chaos," he says.
There was some difficulty finding a workable Plan B. As the project approached a dead-end, it was the book's other illustrator, Charles Raymond - responsible for the colour artwork - who came to the rescue. He volunteered to do the modelling himself, with his German wife, Edeltraud.
Chris Foss, who was responsible for the book's black-and-white illustrations, took the photos. The book's author, Dr Alex Comfort, had given them dozens of positions to get though, and all were done for real over two hectic days in early 1972.
The miners were on strike and they had only limited light to work with before the power cuts would plunge them into darkness.
"We'd say: 'Charlie, we've only got another 20 minutes,'" recalls Mr Foss. "And he'd say: 'Oh I'm terribly sorry' and he'd go off to prepare himself to perform again, and Edeltraud would go: 'Charles, Charles, please, please come on, we only have 10 minutes, please two more positions.'
"So it was all quite fraught shooting the positions - but it worked."
The same kind of matter-of-fact approach applied in the post-production.
"I remember Chris and Charles coming into the office with all these absolutely explicit photographs," says Mr Kindersley.
"And we all stood round saying: 'That's a good one, yeah that's very good.'
"Perhaps we were in a bubble, we were all completely bonkers!" he laughs.
"We just all took it literally. We said: 'Yes, that's a great picture of bondage' or whatever it was.
"There was never the conversation: 'Oh we couldn't put that in the book.'"
The pictures delighted author Alex Comfort, partly because Charles and Edeltraud looked so natural and unposed, absorbed in their private sexual relationship.
"The great thing about Charles and his wife was that they were completely authentic - you couldn't get more authentic," says Mr Kindersley.
"It was a real happening - it wasn't a cooked-up thing - and Alex really liked that."
Dr Comfort had been spending a considerable amount of time in California, whose permissive sexual mores, including "foursomes" and "moresomes", he believed would become the norm everywhere.
He was a frequent visitor to the Sandstone Ranch - a kind of retreat for sexual adventure, where clothes were more often off than on, and sex with multiple partners was the norm.
His message in the book - which marked a big departure from earlier writings on the subject - was that sex, and sexual experimentation, were fun. The different positions reproduced by Charles and Edeltraud were compared to the courses of a meal.
He too, however, was anxious about possible repercussions, such as being struck off the medical register. So he presented himself in the original edition as the editor, claiming that an anonymous couple had handed him the text, which he had merely tweaked and contextualised.
But there were no repercussions. The timing of the book was perfect and the marriage of a sexually liberal message and daring but inoffensive pictures was an instant hit.
As Mr Kindersley toured book fairs around the world, the response was "electric" and many big publishers took it on.
There were of course some rejections. The one he remembers best was the US men's magazine Playboy - the models were just a little too hairy for their liking.
The Joy of Sex ended up selling more than 10 million copies around the world - more than five million in the United States alone, where it stayed in the New York Times best-seller list for a decade.
Photos take over
There had been sex manuals before of course, but they had not been illustrated in anything like such a comprehensive way.
Contrary to popular belief, the Kama Sutra was not originally illustrated, according to James McConnachie, author of The Book of Love: In Search of the Kamasutra, and The Rough Guide to Sex.
The image we have of "a world of exotic, moustachioed aristocrats doing exaggerated acrobatic sex, with women on swings, or with their ankles round their ears" has nothing to do with the original Kama Sutra, says Mr McConnachie.
Miniatures from 16-18th Century India - well over a 1,000 years after the book was written in India - were only tagged on when the text was translated into French and English at the end of the 19th Century, he says.
The Joy of Sex was therefore jumping into untested water, but carried off the illustrations with such aplomb that few have dared even try to emulate it, according to Mr McConnachie.
"The illustrations are fantastic - they are legendary in the history of sex manuals," he says.
Anne Hooper, a British sex therapist and author of numerous bestselling books, agrees.
Like many other sex books in the 1980s, the first ones she wrote were not illustrated at all.
In the years since then there has been "a general easing" in what can be published, she says.
From the 1990s onwards it became common practice - and remains so to this day - to use photography in sex manuals, and once photos were used illustrations suddenly looked dated, says Ms Hooper.
In order to mark a clear distinction with pornography though, genitals are never shown, nor do the models have real sex.
The Joy of Sex is still on sale, but is now a very different book from the original.
Its free-love message sat uneasily with the arrival of HIV/Aids, and Alex Comfort himself revised the text in light of this.
Then, in 2008, the book was significantly updated and re-worked by relationship psychologist Susan Quilliam, who added more of a female perspective on sexuality.
The illustrations were also changed in an attempt to bring it up to date.
"The bearded man was an icon - but he was a '70s icon," says Ms Quilliam.
Chris Foss has not looked at the original black and white illustrations he did for the book for almost 30 years. Snapping open a sturdy little grey suitcase, he starts to root through.
He used to do up to three book covers a week, and so - back then - considered this just another job.
"With the benefit of hindsight, it was a seminal work, but of course at the time, you just didn't realise this."
What does he attribute the book's success to?
He stops and lingers on an image of Charles and Edeltraud, stretched out post-coitally on a rug.
"That's very tender isn't it? They are obviously having a relationship. You can just tell by the way her body lies."
He pauses for a moment. "I think the fact that they were in love had something to do with it."