20 000 000 000 Mark

Stuttgart's city administration issued this twenty-billion-mark note during the most violent phase of the German hyperinflation, when the Reichsbank's own presses could not keep pace with demand and municipalities across Germany became emergency currency issuers almost by necessity. This class of local emergency money — Notgeld — was legally tolerated rather than officially sanctioned, filling a circulation gap that the central monetary authorities had simply lost control of.

The security feature here is nothing more than an administrative stamp. By late 1923, that was the practical ceiling of what local issuers could manage. The denomination itself rendered sophisticated counterfeiting largely pointless.