Marie Casimire Louise de La Grange d'Arquien

Marie Casimire Louise de La Grange d'Arquien, in Polish: Maria Kazimiera, known also by the diminutive form "Marysieńka" (June 28, 1641, Nevers – January 30, 1716, Blois) was consort to King John III Sobieski, from 1674 to 1696.

She came to Poland at the age of five years as a lady in waiting to Marie Louise Gonzaga, the French-born Queen of Poland from 1645 to 1672, wife and consort to two Polish kings — Władysław IV Vasa and later his brother (who succeeded him) John II Casimir Vasa. At the court she met John Sobieski, who arrived there in 1656, but she was first married to Jan "Sobiepan" Zamoyski in 1658, with whom she had three daughters, all died in infancy. Zamoyski died in 1665 and the widowed Marie Casimire eventually married Sobieski on the 14 July the same year. The couple had fourteen children together, but only four of them survived until adult age — Jakub, Aleksander, Konstanty and Teresa (who later became Kurfürstin of Bavaria and mother to Emperor Karl VII).

John Sobieski was elected King of Poland in 1672, not without the influence of his wife. As the Queen of Poland, Marie Casimire supported the proposed Polish–French alliance, while at the same time striving to gain privileges for her family from the French king Louis XIV.

The Royal couple became famous for their love letters, most of which were written from 1665 to 1683, when they were parted either due to John III Sobieski's military engagements or her travels to Paris. The letters give insight not only into the authentic feelings of the loving couple, but also their reflections on contemporary issues and difficulties, as well as down-to-earth matters concerning the royal household and little day-to-day decisions made by the monarch, who often consulted his wife about them. Published long after the death of both of them, they can be credited with popularizing the King's way of addressing the Queen by the very diminutive form of her first name — "Marysieńka". She is widely remembered and referred to in Poland that way.