The Polish-Lithuanian Commonwealth, formally the Kingdom of Poland and the Grand Duchy of Lithuania (also known as the Kingdom of Poland, or just Poland), was a dualistic state, a bi-confederation, of Poland and Lithuania ruled by a common monarch, who was both the king of Poland and the grand duke of Lithuania. It was one of the largest and most populous countries of 16th- and 17th-century Europe. At its peak in the early 17th century, the Commonwealth spanned some 1,200,000 km2) and sustained a multi-ethnic population of 11 million.
The union was formalised by the Union of Lublin in July 1569, but the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania were in a de-facto personal union since 1386 with the marriage of the Polish queen Jadwiga of Poland and Lithuania's Grand Duke Jogaila, who was crowned King jure uxoris Władysław II Jagiełło of Poland. The First Partition of Poland in 1772 and the Second Partition of Poland in 1793 greatly reduced the nation's size and the Commonwealth disappeared as an independent state following the Third Partition of Poland in 1795.
The Union possessed many features unique among contemporary states. Its political system was characterized by strict checks upon monarchical power. These checks were enacted by a legislature (sejm) controlled by the nobility (szlachta). This idiosyncratic system was a precursor to modern concepts of democracy, constitutional monarchy, and federation. The two component states of the Commonwealth were formally equal, yet Poland was the dominant partner in the union.
The Polish-Lithuanian Commonwealth was marked by high levels of ethnic diversity and by relative religious tolerance, guaranteed by the Warsaw Confederation Act 1573; however, the degree of religious freedom varied over time.
After several decades of prosperity, it entered a period of protracted political, military and economic decline. Its growing weakness led to its partitioning among its neighbors, Austria, Prussia and the Russian Empire, during the late 18th century. Shortly before its demise, the Commonwealth adopted a massive reform effort and enacted the Constitution of May 3, 1791 - the first codified constitution in modern European history and the second in modern world history (after the United States Constitution).