December 3: The first custom union is founded in The German Confederation (Deutscher Bund) in 1834 named Zollverein or German Customs Union under the leadership of the Prussian entrepreneurs.

German Zollverein, 1834–1919. Blue: Prussia  in 1834. Grey: Areas included until 1866. Yellow: Austrian possessions outside the Zollverein.Red: Borders of the 1828 German Confederation. 

A coalition of German states led by Prussians reached an agreement to unify custom tariffs and some macroeconomic policies agreed during 1833 Zollverein treaties.

Actually, the customs union talks among german stated led by Prussia began in 1818 when non-Austrian German states agreed to formalized certain customs tariffs among German states. The Zollverein was the very first approach in history to work for an economic union or free tree trade agreements.