Behind the ancient way of life for the Tuareg tribe of the Sahara is a culture so progressive it would even make some in liberal western cultures blush.
Women are allowed to have multiple sexual partners outside of marriage, keep all their property on divorce and are so revered by their sons-in-law that the young men wouldn't dare eat in the same room.
What is even more surprising is that even though the tribe has embraced Islam they have firmly held onto some of the customs that would not be acceptable to the wider Muslim world.
It is the men, and not the women, who cover their faces, for example.
Photographer Henrietta Butler, who has been fascinated by the Tuareg since she first followed them through the desert in 2001, once asked why this was. The explanation was simple.
'The women are beautiful. We would like to see their faces.'
But this is certainly not the only place the Tuareg, related to the Berbers of North Africa, differ from the Muslim world of the Middle East, and even other parts of their own continent.
Before a woman marries, she is free to take as many lovers as she wants.
They turn a blind eye,' explained Butler. 'The young girls have the same great freedoms as the boys.'
For years, the men of the Tuareg have been able to ride to a young woman's tent, and sneak into the side entrance – while his well-trained camel stands quietly and waits.
There, they will spend the night together – while the family, who all live in the tent, politely pretend not to notice.
Should the woman choose to welcome a different man into her tent the next day, so be it.
However, there is also a code of practice which none would dare break. Privacy is all important for this centuries old tribe of nomads, who once crossed the desert bringing dates, salt and saffron south, and slaves and gold north.
he idea of breaking the rules of courtship would be mortifying; as a result, the man is always gone before sunrise.
'The Tuareg are utterly discreet. Everything is done with utmost discretion and respect,' said Butler.
The relaxed customs around sexual partners has resulted in the girls getting married later than they may otherwise do, with the age of 20 not being uncommon.
Although, before then, they will have been wooed with poetry written by the men, who spend hours carefully crafting the words which they hope will win their beloved over.
But it is not a one-way street: the women are just as capable of putting pen to paper, using their own alphabet, taught to them by their mothers.
'The women also make poetry eulogizing the men,' says Butler. 'There is high romance and idolatry.'
Unlike in so many other cultures, women lose none of their power once they marry either.
Many marriages end in divorce among the Tuareg. And when it happens, it is the wife who keeps both the animals and the tent. And it is she who normally decides that she's had enough.
His wife, meanwhile, will keep possession of everything she brought to the marriage and that includes the children.
The mother's camp, Butler explains, is the root of the community, the home everyone returns to – and this arrangement ensures it stays that way.
And there is no shame in divorce. Families will often throw their daughters a divorce party, to let other men know they are available once more.
But this is not a matriarchal society, where the women are in charge.
Butler explains it is still the men 'who sit and talk politics'. But even here, the women can be deferred to. They are often consulted for their views by their sons or husbands, and are quietly pulling the strings behind the scenes.
However, Tuareg society is matri-lineal, which means the families trace their lines through the women, rather than the men, right the way back to their first queen.
So, Butler explained: 'Traditionally, the man would belong to the woman's group, rather than the other way around.'
The preference for the women's line goes as far as man leaving his possessions to his sister's son as it 'is considered a stronger link to your family than to your own son'.
In other words, it can be guaranteed that your sister's child belongs to your sister, rather than a man's son, who cannot be absolutely guaranteed to share his genes.
But there is one tradition which is certainly far more unusual: it is highly rude for a man to eat in front of a woman who he cannot have sexual relations with, or any of his elders.
In front of his mother-in-law it is especially shameful.
'I didn't realize this until the I was having dinner with a Tuareg woman, who had brought her son-in-law as her traveling companion,' Butler recalled.
'We were all sitting down to dinner, and the man has his back turned. She said the poor man was completely horrified because he has to eat with his mother-in-law.'
But it is unlikely he would have ever complained about it, or felt sorry from himself. The very idea is horrendous to the Tuareg.
'You would shame yourself. The Tuareg will go to great lengths to maintain personal dignity. They will suffer,' said Butler.
'If they are not offered water, they won't ask for it – even if they are thirsty.'
Perhaps for this reason, the Tuareg welcome is legendary. They never forget to offer water, and travelers who appear on the horizon will always be 'treated like a king'.