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 that was 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 one of the largest countries in Europe in the 16th and the 17th centuries and had one of the largest populations. The Commonwealth covered more than the present lands of Poland and Lithuania since it also had all of present-day Belarus, a large part of present-day Ukraine and Latvia, and the western part of present-day Russia.