The Polish–Lithuanian Commonwealth, formally the Kingdom of Poland and the Grand Duchy of Lithuania, after 1791 the Commonwealth of Poland, is a dualistic state, a bi-confederation, of Poland and Lithuania ruled by a common monarch, who is both the King of Poland and the Grand Duke of Lithuania.

It is one of the largest and most populous countries of sixteenth- and seventeenth-century Europe.

At its peak in the early seventeenth century, the Commonwealth spans some 450,000 square miles (1,200,000 km2) and sustained a multi-ethnic population of eleven million.

The Commonwealth is established by the Union of Lublin in July 1569, but the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania had been in a de facto personal union since 1386 with the marriage of the Polish queen Hedwig 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 reduces the nation's size and the Commonwealth disappears as an independent state following the Third Partition of Poland in 1795.

The Union possesses many features unique among contemporary states.

Its political system is characterized by strict checks upon monarchical power.

These checks are enacted by a legislature (sejm) controlled by the nobility (szlachta).

This idiosyncratic system is a precursor to modern concepts of democracy, constitutional monarchy, and federation.

Although the two component states of the Commonwealth were formally equal, Poland was the dominant partner in the union.

The Polish–Lithuanian Commonwealth is 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 enters a period of protracted political, military and economic decline.

Its growing weakness lead to its partitioning among its neighbors, Austria, Prussia and the Russian Empire, during the late eighteenth century

Shortly before its demise, the Commonwealth adopts a massive reform effort and enacts 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).