Treaty of Wehlau, in Encyclopædia Britannica, 2009. URL consultato il 23 maggio 2009.
«(Sept. 19, 1657), agreement in which John Casimir, king of Poland from 1648 to 1668, renounced the suzerainty of the Polish crown over ducal Prussia and made Frederick William, who was the duke of Prussia as well as the elector of Brandenburg (1640–88), the duchy’s sovereign ruler.»