Sigismund I the Old in the context of "Sigismund II Augustus"

Casimir Jagiellon (/ˈkæzɪmɪər/; Latin: Casimirus; Lithuanian: Kazimieras; Polish: Kazimierz; 3 October 1458 – 4 March 1484) was a prince of the Kingdom of Poland and of the Grand Duchy of Lithuania. The second son of King Casimir IV Jagiellon, he was tutored by Johannes Longinus, a Polish chronicler, diplomat, and priest. After his elder brother Vladislaus was elected as King of Bohemia in 1471, Casimir became the heir apparent. At the age of 13, Casimir participated in the failed military campaign to install him as King of Hungary. He became known for his piety, devotion to God, and generosity towards the sick and poor. He became ill (most likely with tuberculosis) and died at the age of 25. He was buried in Vilnius Cathedral. His canonization was initiated by his brother King Sigismund I the Old in 1514 and the tradition holds that he was canonized in 1521. Saint Casimir the Prince is the patron saint of Poland, Lithuania and Lithuanian youth and the only saint with this name.

Veneration of Casimir saw a resurgence in the 17th century when his feast day was confirmed by the pope in 1602 and the dedicated Chapel of Saint Casimir was completed in 1636. Casimir became a patron saint of Lithuania and Lithuanian youth. In Vilnius, his feast day is marked annually with Kaziuko mugė (a trade fair) held on the Sunday nearest to 4 March, the anniversary of his death. There are more than 50 churches named after Casimir in Lithuania and Poland, including Church of St. Casimir, Vilnius and St. Kazimierz Church, Warsaw, and more than 50 churches in Lithuanian and Polish diaspora communities in America. Women's congregation Sisters of Saint Casimir was established in 1908 and remains active in the United States.

Barbara Zápolya (Hungarian: Szapolyai Borbála, Lithuanian: Barbora Zapojajietė, 1495–1515) was Queen of Poland and Grand Duchess of Lithuania as the first wife of Sigismund I the Old from 1512 to 1515. Marriage to Barbara represented an alliance between Sigismund and the House of Zápolya against the Habsburgs in succession disputes over the throne to the Kingdom of Hungary. The alliance was short-lived as the renewed Muscovite–Lithuanian War forced Sigismund to look for Habsburg allies. The marriage was loving, but short. Barbara was the mother of Hedwig, Electress of Brandenburg, but died soon after the birth of her second daughter Anna.

Isabella Jagiellon (Hungarian: Izabella királyné; Polish: Izabela Jagiellonka; Lithuanian: Izabelė Jogailaitė; 18 January 1519 – 15 September 1559) was a princess of the Kingdom of Poland and of the Grand Duchy of Lithuania, and later also the Queen consort of Hungary. She was the oldest child of Sigismund I the Old, King of Poland and Grand Duke of Lithuania, and his Italian wife Bona Sforza.

In 1539, she married John Zápolya, Voivode of Transylvania and King of Hungary. At the time Hungary was contested between Ferdinand of Austria who wanted to add it to the Habsburg domains (see Royal Hungary), local nobles who wanted to keep Hungary independent (see Eastern Hungarian Kingdom), and Sultan Suleiman the Magnificent who saw it as a vassal state of the Ottoman Empire (see also Little War in Hungary). While Isabella's marriage lasted only a year and a half, it did produce a male heir – John Sigismund Zápolya born just two weeks before his father's death in July 1540.

Albert of Prussia (German: Albrecht von Preussen; 17 May 1490 – 20 March 1568) was a German prince who was the 37th grand master of the Teutonic Knights and, after converting to Lutheranism, became the first ruler of the Duchy of Prussia, the secularized state that emerged from the former Monastic State of the Teutonic Knights. Albert was the first European ruler to establish Lutheranism, and thus Protestantism, as the official state religion of his lands. He proved instrumental in the political spread of Protestantism in its early stage, ruling the Prussian lands for nearly six decades (1510–1568).

Albert was great-grandson of the converted pagan ruler Jogaila of Poland and Lithuania, vanquisher of the Teutonic Knights at the Battle of Grunwald. He was also a member of the Brandenburg-Ansbach branch of the House of Hohenzollern. He became grand master of the Teutonic Knights in their attempt to diplomatically win over the Polish-Lithuanian union. His skill in political administration and leadership ultimately succeeded in reversing the decline of the Teutonic Order. But Albert was sympathetic to the demands of Martin Luther, whose teachings had become popular in his lands. So he rebelled against the Roman Catholic Church and the Holy Roman Empire by converting the Teutonic state into a Protestant and hereditary realm, the Duchy of Prussia, for which he paid homage to his uncle, Sigismund I, king of Poland. That arrangement was confirmed by the Treaty of Kraków in 1525. Albert pledged a personal oath to the king and in return was invested with the duchy for himself and his heirs.

The Polish–Teutonic War of 1519–1521 (German: Reiterkrieg, lit. 'Rider's War', Polish: Wojna pruska, lit. 'Prussian War') was fought between the Kingdom of Poland and the Teutonic Knights, ending with the Compromise of Thorn in April 1521. Four years later, under the Treaty of Kraków, part of the Catholic Monastic State of the Teutonic Order became secularized as the Duchy of Prussia. The reigning Grand Master Albert of Hohenzollern-Brandenburg-Ansbach became the first Duke of Prussia by paying the Prussian Homage as vassal to his uncle, Polish king and grand Duke of Lithuania, Sigismund I the Old (1467–1548, reigned 1506–1548).

Hedwig Jagiellon (Polish: Jadwiga Jagiellonka, Lithuanian: Jadvyga Jogailaitė, German: Hedwig Jagiellonica; 15 March 1513 – 7 February 1573) was a princess of the Kingdom of Poland and of the Grand Duchy of Lithuania, and a member of the Jagiellonian dynasty as a daughter of Sigismund I the Old. She was Electress of Brandenburg by marriage to Joachim II Hector, Elector of Brandenburg.