the polish–lithuanian commonwealth – formally, the crown of the kingdom of poland and the grand duchy of lithuania and, after 1791, the commonwealth of poland – was a dual state, a bi-confederation of poland and lithuania ruled by a common monarch, 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 and sustained a multi-ethnic population of 11 million. polish and latin were the two co-official languages.