The purity or fineness of silver alloys is now described using the millesimal system in most countries. This system uses a number to represent the purity of the alloy. The number described purity in parts per thousand.

Previous to the millesimal system, the fineness of silver was expressed in carats. While the fineness of silver alloys must be stamped or hallmarked into pieces, the millesimal value is generally compulsory and the carat value now optional.

Table 1. Some of the most common fineness denominations used.

Fineness	Common Name	Purity (wt.%)
999	Fine or pure silver	99.9
958	Brittania	95.8
925	Sterling silver	92.5
800	Jewellery silver	80

Pure silver is typically very soft and malleable, hence it is commonly alloyed to increase its hardness and durability for applications such as jewellery. It is typically alloyed with copper in this instance, with sterling silver being one of the most popular alloys, containing 7.5% copper. Copper is used as it is a hardening agent and does not discolour the silver.