From medieval times to the French Revolution, royal mistresses inhabited a world of unrivalled glamour and luxury.
Falling out of favour with the king cost Anne Boleyn her head
It was also a dangerous one beset by pitfalls, in the worst cases death, and sometimes hardship, but for the lucky few it opened the door to great personal wealth, security and a rare chance of political power.
From the glamorous Madame de Pompadour to Nell Gwynn and Wallis Simpson, for centuries kings across Europe have turned to mistresses for sex, advice and conversation.
Author Eleanor Herman, who has spent years researching royal mistresses for her book, Sex With Kings, says as long as there have been kings, there have probably been mistresses.
But their heyday was in the 16th, 17th and 18th centuries, when having one became almost a requirement for a French, English or German king.
Elephant and Castle
From the early 1500s, mistresses were given official titles, generous allowances and even pensions.
Though many were beautiful, some were known for their ugliness.
George I of England had two unattractive mistresses, one known as the Maypole, because she was so tall and thin, and a short, fat one nicknamed Elephant and Castle.
Neither was having a mistress all about sex.
Madame de Pompadour remained King Louis XV's mistress for 19 years in France, despite being unable to have intercourse.
Instead, she contracted that aspect of the relationship out, providing the king with young women to sleep with.
Ms Herman says: "After a hard day, the king wanted to come to his mistress' rooms and find a decent dinner waiting.
"He wanted someone to ask about his day, tell him the court gossip and make him laugh. It was about being good company."
In this way, Prince Charles' mistress Camilla Parker Bowles, who he married earlier this month, followed in the footsteps of many a royal mistress - succeeding where the younger Princess Diana failed, says Ms Herman.
Wallis won her man but lost the crown
Ms Herman says: "Diana was beautiful but she was emotionally very needy.
"She would nag Charles, berate him and insult him and then he'd visit Camilla, who would give him wine and cheese and tell him a funny story. That was the difference."
Successful mistresses became fabulously wealthy, securing the future prosperity of their families. Some also gained great political power.
In the 1540s and 50s, the French King Henri II's mistress, Diane de Poitiers, imposed taxes, appointed ministers and made laws.
Gabrielle d'Estrees, the Catholic mistress of Protestant Henri IV, was instrumental in ending France's religious wars.
Because of the rewards on offer, competition for the job of royal mistress could be intense.
Charles II of England had several at once, forcing them to spend their time vying to be the favourite and verbally stabbing each other in the back.
The most famous was the actress Nell Gwynn, who once sabotaged a rival's rendezvous with the king by inviting her to lunch beforehand and putting laxatives in her food.
In France, Athenais de Montespan wanted so much to be Louis XIV's mistress that she secretly gave him love potions made from babies' intestines, toads and bats.
However, once she had snared her man, a king's mistress could not be complacent.
Ms Herman says: "You weren't allowed to be sick - you had to drag yourself out of bed, put rouge on your cheeks and diamonds in your ears and put on a smile.
"The moment you were cross, weren't feeling well or couldn't entertain him, there would be someone else waiting to take your place."
Other pitfalls included death in childbirth or venereal disease.
After all their hard work, some mistresses were able to retire rich in their 30s or 40s, marry who they chose and live happily ever after.
Diana was "beautiful but needy"
Others, such as Anne Boleyn, mistress and then wife of England's Henry VIII, were not so lucky.
She was famously executed in 1536 after falling out of favour with the king.
And in Bavaria, the hedonistic lifestyle of Ludovic I's Irish mistress Lola Montez earned her such public hatred that she was run out of the country.
The excessive riches and luxurious lifestyles of Europe's royal mistresses eventually spelled their downfall, as an increasingly fed-up public started to demand a change.
After the French revolution in 1789, a new climate of morality meant kings had to keep their extra-marital relations strictly secret.
In 1936, Britain's Edward VIII was forced to abdicate to be with his mistress - the divorcee Wallis Simpson.
Ms Herman says she died a "disappointed woman" having won her man but lost the crown.
But in the case of Charles and Camilla, Ms Herman foresees a more favourable future, saying: "I think they'll be happy, and that's ultimately winning, isn't it."
Sex With Kings by Eleanor Herman is published by Harpercollins Publishers.