Congruent with the Kingdom of Prussia proper (i.e. former Ducal and Royal Prussia), its territory, like the Greater Polish territory of Posen, was not part of the German Confederation.
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They were established in the course of the Stein-Hardenberg Reforms from 1815 to 1818 at an intermediate level, between the higher provinces and the government districts (Regierungsbezirke), and the lower municipal governments (Gemeinden).
The flag is a combination of the two former provinces of Prussia that comprise most of the state: the Rhine Province and Westphalia, which unfortunately can easily be confused with the much older Flag of Hungary, but which is inverted relative to the NRWF flag, and with the plain version (not currently used) of the flag of Iran.
Besides the smaller Protestant denominations of the Mennonites, Baptists and Methodists, which were organised crossing state borders along denominational lines, there were 29 (later 28) church bodies organised according to the territorial borders of German states or Prussian provinces.