Formally known as the Kingdom of Poland and the Grand Duchy of Lithuania, and, after 1791, as the Commonwealth of Poland, was a country and bi-federation of Poland and Lithuania ruled by a common monarch in real union, who was both King of Poland and Grand Duke of Lithuania. 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 km2 (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.

Polish–Lithuanian Commonwealth - Wikipedia