经济学家将金属货币的类型分为两大类：单一金属本位和双金属本位。前者是指主要货币仅限于一种金属铸币。至于后者，即双金属本位，金币和银币都作为主要货币。双金属本位要求具备三个条件。

Economists divide the types of metallic currencies in to two main types: the single metallic standard and the dual metallic standard. The first is where the main currencies are restricted to one single metallic coinage. As for the latter i.e. the dual standard, both the gold and silver coins represent the main currency. The dual metallic standard requires the existence of three qualities:

1. 金币必须是无限制的法定货币（不具有固定的购买力）。

1. Gold coins must have an unrestricted legal tender (no fixed purchasing power).

2. 对于两种金属的金属锭（生金银），铸造时不应有任何限制。

2. There should be no restrictions on minting from the bullion of both metals.

3. 必须确定金币与银币价值之间的官方兑换比率。

3. An official rate between the values of the gold and silver coins must be established.

双金属本位制的特点是流通货币数量庞大，因为金币和银币同时作为主要货币使用。因此，物价将保持在较高水平，这会促使生产增长。这也会使货币价值更加稳定，物价大幅波动的可能性随之降低，而物价大幅波动通常会引发经济动荡。由此可见，实行双金属本位制优于单金属本位制。

The dual metallic standard is characterised by the huge amount of money it puts into circulation, due to the simultaneous use of the metallic coins as main currencies. Therefore, prices remain high and this would lead to an increase in production. This would also make the value of money more stable and prices would be less likely to undergo major fluctuations which usually leads to economic unrest. It is therefore clear that operating a dual metallic standard is better than the single metallic standard.