In 1618, the Prussian Hohenzollern were extinct in the male line, and so the Polish fief of Prussia was passed on to the senior Brandenburg Hohenzollern line, the ruling margraves and prince-electors of Brandenburg, who thereafter ruled Brandenburg (a fief of the Holy Roman Empire), and Ducal Prussia (a Polish fief), in personal union. This legal contradiction made a cross-border real union impossible; however, in practice, Brandenburg and Ducal Prussia were more and more ruled as one, and colloquially referred to as "Brandenburg-Prussia".