Article: The debate on a new delimitation of the German territory started in 1919 as part of discussions about the new constitution. Hugo Preuss, the father of the Weimar Constitution, drafted a plan to divide the German Reich into 14 roughly equal-sized states. His proposal was turned down due to opposition of the states and concerns of the government. Article 18 of the constitution enabled a new delimitation of the German territory but set high hurdles: Three fifth of the votes handed in, and at least the majority of the population are necessary to decide on the alteration of territory. In fact, until 1933 there were only four changes in the configuration of the German states: The 7 Thuringian states were merged in 1920, whereby Coburg opted for Bavaria, Pyrmont joined Prussia in 1922, and Waldeck did so in 1929. Any later plans to break up the dominating Prussia into smaller states failed because political circumstances were not favorable to state reforms.

Question: When did Pyrmont join Prussia?
1922