The Polish–Lithuanian Commonwealth, also referred to as Poland–Lithuania or the First Polish Republic (Polish: I Rzeczpospolita), was a federative real union between the Kingdom of Poland and the Grand Duchy of Lithuania, existing from 1569 to 1795. This state was among the largest, most populated countries of 16th- to 18th-century Europe. At its peak in the early 17th century, the Rzeczpospolita spanned approximately 1,000,000 km2 (390,000 sq mi) and supported a multi-ethnic population of around 12 million as of 1618. The official languages of the Commonwealth were Polish and Latin, with Catholicism as the state religion. The Union of Lublin established the Commonwealth as a single entity on 1 July 1569. The two nations had previously been in a personal union since the Krewo Agreement of 1385 (Polish–Lithuanian union) and the subsequent marriage of Queen Jadwiga of Poland to Grand Duke Jogaila of Lithuania, who was crowned as Władysław II Jagiełło, 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. It 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, were an early example of constitutional monarchy. The General Sejm, the bicameral Parliament, held legislative power; its lower house was elected by szlachta nobles comprising some 15% of the population. A constitutional statute, the Henrician Articles, bound the king and his government, 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 found itself under sustained, combined assault from its neighbours and entered a period of protracted political and military 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 modern codified constitution in European history and the second in world history after the United States Constitution. Source: Wikipedia (en)