Imperial and US customary measurement systems different concepts – the imperial gallon is defined in terms of the volume occupied by a specified mass of water, while the US gallon is specified in terms of cubic linear measure. Notes Notes Units of weight in both the imperial and US customary system always used the same standard, though differences in multiples of the avoirdupois pound developed in the nineteenth century. Both systems used the three different scales – the avoirdupois system for general use, the troy system for precious metals and the apothecaries system in the pharmacy industry. The term "weight" and "mass" are used interchangeably in
Coinage metals well known in the Persian and Sassanids era. Most notably, in Susa and in Ctesiphon. However, there are many more, even for coins made from intrinsically precious metals. Precious metals are always used in bullion coins and some collectable coins. Coins not intended for circulation or for intrinsic value have also been made experimentally using an even larger variety of metals, since they function as fiat money. In general coins intended for circulation must have metal values considerably less than their face values, for reasons discussed below. Coins that are intended for circulation have some special requirements based on the