The Polish–Lithuanian Commonwealth, formally known as the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania and, after 1791, the Commonwealth of Poland, was a state of Poland and Lithuania ruled by a common monarch. The Commonwealth was an extension of the Polish-Lithuanian Union, a personal union between those two states that had existed from 1386. It was the largest and one of the most populous countries of 16th- and 17th-century Europe. The Polish-Lithuanian Commonwealth covered more lands than the present lands of Poland and Lithuania. The lands of the Commonwealth also covered all the lands of present-day Belarus; a large part of Ukraine and Latvia; and western part of present-day Russia.