notes and coins of cheaper metal. As the war dragged on, acute monetary shortages could not be met by the German central bank, leading to a new period of Notgeld beginning in 1916. Additionally, the non-precious metals used to mint lower value coins were needed to produce war supplies. Dr. Keller arranged this period into two catalogs: Kleingeldscheine for issues of less than 1 Mark face value and Grossgeldscheine for values 1 Mark and higher. This period of issue came to a close in 1919. World War I prison camp money Although camp money used by prisoners of war was different from Notgeld, collectors inevitably lumped this material into the hobby. The period covered the entire war, 1914–1918. This field of collecting may include World War II issues, though this covers only notes circulated in concentration camps, as the German Luftwaffe in charge of prisoners of war prepared a general issue of notes for all camps under their direction. Collector series Though the production of Notgeld was initially amateurish, with many set by typewriter or even handwritten, collectors soon appeared on the scene to take hold of the expired 1914 stock. With the next wave of issues in the latter half of the war, Notgeld production was handled by professional printers. These issues incorporated pleasing designs, and a new reason for hoarding came into being. As the issuing bodies realized this demand, they began to issue notes in 1920, well after their economic necessity had ended. They may have been motivated by the success of Austrian collector Notgeld earlier in the year (see below). Notes were issued predominantly in 1921 and were usually extremely colorful. These depicted many subjects, such as local buildings, local scenery and folklore, as well as politics. Many were released in series of 6, 8, or more notes of the same denomination, and tell a short story, with often whimsical illustrations. Often, they were sold to collectors in special envelope packets printed with a description of the series. Dr. Keller published information on releases in his magazine Das Notgeld. Often, he used his publication to criticize issuers for charging collectors more money for the series than their face value. These collector-only sets, which were never intended to circulate, were known as Serienscheine (pieces issued as a part of a series). Quite often, the validity period of the note had already expired when the Notgeld was issued.