Diplomats who wanted to get to the King Louis XV in the mid-18th century had to go through Madame de Pompadour, who would meet with them while applying her makeup in public. Though Pompadour’s official title was as the French king’s mistress, she only slept with him for the first several years of their 20-year relationship. For the rest of it, she was the king’s closest political advisor and confidant.

Pompadour was the first woman to hold an official title of maîtresse-en-titre, or official mistress to the King of France (and since the monarchy fell with the next king, she was also one of the last). Yet her title reflected a role that mistresses had played in European kingdoms for centuries as senior political figures in the king’s court. While this role was less formal in England than it was in France, in both monarchies the person with the most influence over the king’s decisions was whoever had his ear. And what else is pillow talk?

“There’s not a real division between formal and informal political power in the early French court,” says Christine M. Adams, coauthor with Tracy Adams of the forthcoming book, The Creation of the Official French Royal Mistress. “If you were a friend”—or more than a friend—“that makes you politically influential. You can get favors for your friends. You can get land. You can get money.”

It was pretty common for kings to have a mistress in those days, in part because marriages were arranged for political gain and not personal companionship. “They would often be paired with someone who they may not have known very well or they may not have liked,” says Danièle Cybulskie, author of the forthcoming book Life in Medieval Europe: Fact and Fictions. Adultery was still frowned upon, and kings could be deposed if they appeared to act too immorally, but people mostly tolerated a king having one mistress at a time.

This didn’t mean the queen got to have a boyfriend, though. This was considered treasonous because it created uncertainty about whether any children she had were rightful heirs. Even the unmarried Queen Elizabeth I was more private about having gentlemen suitors than many kings were about their mistresses. Same-sex affairs were even more taboo, and rumors of them created problems for the English royals King Edward II and Queen Anne (as depicted in The Favourite).

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Most members of a king’s court would’ve known who his mistress was, and likely been jealous and suspicious of her influence. Yet members of the court could also advance their own interests by winning a mistress’s favor. In the 17th century, Barbara Palmer helped men like Henry Bennet, 1st Earl of Arlington, gain political access to her companion Charles II, King of England. She also helped secure official titles for some of her illegitimate children with the king.

Depending on the country and time period, regular people outside of the court might know who the king’s mistress was, too. Kathleen A. Wellman, author of Queens and Mistresses of Renaissance France, says it was common for the king to travel around France in the 15th and 16th centuries and present himself to the people in public ceremonies with his mistress instead of his wife.

There were different reasons why the king might do this. “The queen might be pregnant, or the mistress might be more attractive or it might be easier to suggest certain things about the king by using the mistress,” Wellman says. What kind of things? Well, “kings liked to present themselves…as allegories, suggesting that they are like other famous people, whether those people are saints or Greek and Roman gods.”

As an example, she points to Henry II, who was the King of France in the mid-16th century. Henry II’s mistress was named Diane de Poitiers, and he often presented her in a way that suggested she was the Roman goddess Diana and he was a god beside her. If this sounds a little too abstract, just think about how Jackie Kennedy framed JFK’s presidency in a Life magazine profile after his death: “There will be great presidents again, but there will never be another Camelot.”

In fact, Wellman says presidential first ladies offer an apt analogy for the role that royal mistresses played. “Think about the influence that first ladies have in shaping perceptions of presidencies,” she says. “And think about all the people who had to go through Nancy Reagan to get to Ronald Reagan.”

Just as a first lady might take some of the political blame for aspects of her husband’s presidency, so too might a mistress receive blame for the king’s decisions. This was especially true of Madame de Pompadour, the first woman to hold the title of the king’s mistress (after she died, Louis XV gave the title to Madame du Barry). Her visibly elevated role opened her up to even more criticism than mistresses before her, says Adams. In fact, if Marie-Antoinette’s husband Louis XVI had had a mistress, she might have shielded Marie-Antoinette from some of the attacks she received, particularly the accusations of supposed immorality, which were normally reserved for mistresses.

“For too long, because historians have tended to look at formal political structures...they’ve ignored the political role of mistresses,” Adams says. “[The mistress] is completely dependent on the king for her power, and that means that she’s going to look out for his interests alone. And that’s one of the reasons that she is so powerful.”

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