=Free Coinage.=--In determining its coinage policy, the government might follow either of two methods: (1) It might coin any and all bullion presented by its owners at the mints, or (2) it might purchase its own bullion and coin only so much as the necessities of trade or other considerations might require. The former policy is that of free coinage; it is also unlimited coinage since it involves the coinage of all bullion offered, without limit. From the very first, the practice of the government in regard to gold has been that of free and unlimited coinage; that is, any owner of gold bullion may take it to a mint and have it coined without charge except for the cost of the alloy. Prior to 1873, the same policy was followed in regard to silver, thus maintaining in theory at least a bimetallic or double standard. In 1873, however, Congress abandoned the policy of free coinage of silver and adopted the single gold standard. From then until now the government has coined no silver bullion for private owners. =Paper Currency.=--In addition to the metallic money described above there is a vast amount of paper currency in the United States. This currency may be classified under five different heads. _Greenbacks._--First, there are the $346,681,016 of old United States notes or "greenbacks," already described. They were issued during the Civil War, they bear no interest, and are redeemable in coin upon the demand of the holder. Since 1878 the practice of the government has been not to retire them as they are redeemed but to reissue them and keep them in circulation. _Gold and Silver Certificates._--Second, there is a large amount of currency in the form of gold and silver certificates. The law under which such currency is issued provides that any owner of gold or silver coin may deposit it in the treasury and receive in exchange an equivalent amount of certificates. They are more convenient to handle than coin, and are equally valuable for paying debts and purchasing commodities. On the 1st of July, 1921, the amount of gold certificates in circulation was $452,174,709; the amount of silver certificates, $201,534,213. These two forms of currency constitute one eighth of our entire stock of money in circulation.