William I, or Wilhelm I, was King of Prussia from 1861 and German Emperor from 1871 until his death in 1888. A member of the House of Hohenzollern, he was the first head of state of a united Germany. He was de facto head of state of Prussia from 1858, when he became regent for his brother Frederick William IV. During the reign of his grandson Wilhelm II, he was known as Wilhelm the Great.

The Landtag of Prussia was the representative assembly of the Kingdom of Prussia implemented in 1849, a bicameral legislature consisting of the upper House of Lords (Herrenhaus) and the lower House of Representatives (Abgeordnetenhaus). After World War I and the German Revolution of 1918–19 the Landtag diet continued as the parliament of the Free State of Prussia between 1921 and 1934, when it was abolished by the Nazi regime.