King in Prussia
Prussia was transformed from a duchy to a kingdom on 18 January 1701 by Elector Frederick III of Brandenburg who declared himself King in Prussia. He was allowed to be king only in the Prussian kingdom to avoid offending the Holy Roman Emperor Leopold I, who had power over most of his lands. Brandenburg was treated as a separate state from the Prussian Kingdom. The state of Brandenburg was later considered as part of Prussia although most of its territories lay outside Prussia proper. The kingdom grew magnificently during King Frederick I reign.