1490–1568, grand master of the Teutonic Knights (1511–25), first duke of Prussia (1525–68); grandson of Elector Albert Achilles of Brandenburg. In 1525 he became a Protestant, and on the advice of Martin Luther he secularized the dominions of the Teutonic Knights and became duke of the hereditary duchy of Prussia. The knights' lands had been held as a fief from the king of Poland, and the new duchy remained under Polish suzerainty. On the extinction of Albert's line (1618), Prussia passed to the senior line of Brandenburg, and in 1701 it was made a kingdom.