During the early 20th century special money was issued in several countries, but primarily Germany and Austria to cater for economic crisis situations. There was a shortage of small change, due to the war efforts and the need for metal going that way rather than for coinage. This emergency money was not issued by the central bank (Reichsbank), but by various other institutions, e.g. town savings banks, municipalities, private and state-owned firms. It was therefore not a legal tender but rather a mutually accepted means of payment, in particular location or site.
Notgeld was mainly issued in the form of (paper) banknotes. Sometimes other forms were used, as well: coins, leather, silk, linen, stamps, aluminium foil, coal and all sorts of re-used paper and carton material (e.g. playing cards).
As these banknotes were very colorful, they soon became a target for collectors. As the issuing bodies realised this demand they continued to issue these notes beyond their economic necessity up till 1922.