The Swiss franc (plural: francs; in German: Frank, plural: Franken) has been the currency of Liechtenstein since 1920. The Swiss franc is legal tender since Liechtenstein is in a customs and monetary union with Switzerland. The 1980 treaty between Switzerland and Liechtenstein allows Liechtenstein to mint limited amounts of Swiss francs with a Liechtenstein inscription, but only in the form of commemorative coins (mainly issued for collectors), and they are not allowed to issue banknotes. Liechtenstein used the Austrian krone and heller (and, of course, the Liechtenstein krone) until 1920, and switched to the Swiss franc due to the krone's instability. Liechtenstein coins are so rare that they do not actually circulate, and no banknotes have been issued, with the exception of three emergency issues of heller in 1920. Most of the Liechtenstein franc coins have the same amount of precious metal as the Swiss franc, except for the coins minted in the late 1980s and 1990s. The highest number of Liechtenstein franc coins minted was the 1 franc minted in 1924; 60,000 were struck, but 45,355 of them were later melted. If the number of melted coins is excluded, the highest mintage would be the 50 franc piece minted in 1988 commemorating the 50th anniversary of the reign of Prince Franz Joseph II and the 10 franc piece minted in 1990 commemorating the succession of Prince Hans-Adam II. Both number 35,000. History of early currencies used in Liechtenstein The following denominations were issued 1728: 20 kreutzer, thaler, 1 thaler, 1 ducat and 10 ducats 1758: thaler, 1 thaler and 1 ducat 1778: thaler, 1 thaler and 1 ducat The ducats were in minted in 986 gold, and all other coins were minted in 583 silver. All coins bore on the obverse side the right-facing bust of each prince and on the reverse side his arms. In 1862, under Prince Johann II, another coin was minted, a vereinsthaler, which had the same design as the previous coins and was taken out of circulation in 1893, with a value of 3.53 crowns. There was a currency reform on 26 August 1898. 1 florin (German: gulden) now had the value of a Liechtenstein crown (Liechtenstein krone) and there were 100 hellers in a crown. Under Johann II, silver coins were issued with denominations of 1 crown, 2 crowns and 5 crowns. Coins with denominations of 10 crowns and 20 crowns were minted