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Poland–Lithuania, formally known as the Kingdom of Poland and the Grand Duchy of Lithuania and also referred to as the Polish–Lithuanian Commonwealth or the First Polish Republic, was a federative real union of the Kingdom of Poland and the Grand Duchy of Lithuania between 1569 and 1795. It was one of the largest and most populous countries of 16th- to 17th-century Europe. At its largest territorial extent, in the early 17th century, the Commonwealth covered almost 1,000,000 km (400,000 sq mi) and as of 1618 sustained a multi-ethnic population of almost 12 million. Polish and Latin were the two co-official languages. Catholicism was the state religion, though religious freedom was established in 1573.

The Commonwealth was established as a single entity by the Union of Lublin on 1 July 1569. The two nations had previously been in a personal union since the Krewo Agreement of 1385 and the subsequent marriage of Queen Jadwiga of Poland to Grand Duke Jogaila of Lithuania, who was crowned jure uxoris King of Poland. Their descendant, Sigismund II Augustus, enforced the merger to strengthen frontiers of his dominion and maintain unity as he remained childless. His death in 1572 marked the end of the Jagiellonian dynasty and introduced an elective monarchy, whereupon members of domestic noble families or external dynasties were elected to the throne for life.

The Commonwealth's parliamentary system of government and elective monarchy, called the Golden Liberty, was an early example of constitutional monarchy. The General Sejm, the bicameral Parliament, held legislative power; its lower house was elected by all szlachta (some 15% of the population). The king and his government were bound by a constitutional statute, the Henrician Articles, which tightly circumscribed royal authority. The country also exhibited unusual levels of ethnic diversity and great religious tolerance by European standards, guaranteed by the Warsaw Confederation Act of 1573, though the practical degree of religious freedom varied. Poland acted as the dominant partner in the union. Polonization of nobles was generally voluntary, but state efforts at religious conversion were sometimes resisted.

After a long period of prosperity, the Commonwealth entered a period of protracted political, military, and economic decline. Its growing weakness led to its partitioning among its neighbours, Austria, Prussia, and Russia, during the late 18th century. Shortly before its demise, the Commonwealth adopted a major reform effort and enacted the 3 May Constitution, which was the first codified constitution in modern European history and the second in modern world history after the United States Constitution.