 In other words, while he was a king in Prussia, he was still only an elector under the suzerainty of the Holy Roman Emperor in Brandenburg.Legally, the Hohenzollern state was still a personal union between Brandenburg and Prussia.However, by the time Frederick crowned himself as king, the emperor's authority over Brandenburg (and the rest of the empire) was only nominal, and in practice it soon came to be treated as part of the Prussian kingdom rather than as a separate entity.