The First Female Monarch Crowned King

Jadwiga, the youngest daughter of Louis I the Great, became the ruler of Poland on October 16, 1384. Thanks to a quirk in Polish law, however, the young girl, also known as Hedwig, was crowned a king, not a queen. She is one of only approximately five women in history to take on the masculine title.
A Regal Twist Of Fate
As the king's youngest daughter, it was never expected that Jadwiga would become Poland's ruler. Louis's eldest daughter, Catherine, was expected to one day rule the throne of Poland and Hungary, but Catherine ended up predeceasing her father. The throne was then supposed to go to the second-oldest daughter, Mary, but Poland had decided against continuing the Personal Union of Crowns with Hungary and refused to accept Mary's fiance, Sigismund, future Holy Roman Emperor and King of Hungary. The next choice was Jadwiga, age 10 at the time, who traveled from her native Hungary to Poland, where she was crowned at the Wawel Cathedral in Krakow. According to Polish law, the country had to have a king, but it didn't say the king had to be a man. Instead of rewriting the law, and to make it clear that she was not just a queen consort, little Hedwig was crowned king.

The Girl King
Regardless of her tender age, Jadwiga was an impressive leader. She was well educated, and she spoke at least six different languages, including Polish, Latin, Hungarian, Serbian, Bosnian, and German. Jadwiga's mother, Elizabeth of Bosnia, was supposed to serve as regent until Jadwiga came of age, but due to Jadwiga's surprising maturity and leadership acumen, it was decided a regency wasn't necessary. She was so charitable, funding scholarships and new hospitals, that she was venerated by the Roman Catholic Church as the patron saint of queens. She even donated her crown jewelry to the restoration and modernization of Krakow University.

Married At 12
As soon as Jadwiga's coronation was over, the selection of her future husband became the country's most important debate. Jadwiga wanted to marry her childhood friend, William of Austria, the son of Leopold III and Viridis Visconti; in fact, they were already unofficially engaged, but the nobles had other plans. It was determined that a marriage to Jogaila, King of Lithuania, would be most beneficial for Poland, so she did what was best for her land, declared her engagement to William invalid, and married the 26-year-old king.

Family Tragedies
Jadwiga's marriage turned out to be a very successful union, full of respect and devotion, by all accounts. Even though Jogaila became king, Jadwiga remained a ruler and executed her powers whenever necessary. Still, Jadwiga's reign was difficult at times. The opposition with Hungary ended with her mother and sister being kidnapped and imprisoned, whereupon Jadwiga's mother was strangled before her eldest surviving daughter's eyes. Mary was eventually rescued but, a few years later, unfortunately passed away while pregnant in a horse-riding accident.
In 1398, Jadwiga and Jogaila announced their first pregnancy, and Jadwiga gave birth to her daughter, Elizabeth, on June 22, 1399. Elizabeth was expected to eventually rule Poland, Lithuania, and Hungary, but she never even reached her first month of life, and her mother soon followed. They were buried together in Wawel Cathedral, and Jadwiga was immediately canonized as a saint.