It is used to measure the purity of gold alloys. One karat (or carat) is equal to 1/24 purity by mass.

• 24 carat (millesimal fineness 99.9% or higher)purity
• 22 carat (millesimal fineness 91.6%)purity
• 21 carat (millesimal fineness 87.5%)purity
• 20 carat (millesimal fineness 83.3%)purity
• 18 carat (millesimal fineness 75.0%)purity
• 15 carat (millesimal fineness 62.5%)purity
• 14 carat (millesimal fineness 58.5%)purity
• 10 carat (millesimal fineness 41.7%)purity
• 9 carat (millesimal fineness 37.5%)purity
• 8 carat (millesimal fineness 33.3%)purity
• 1 carat (millesimal fineness 04.2%)purity

For the benefit of those who do not know what a karat is, the karat (a.k.a. carat) is a measure of the purity of gold alloys. There is no such thing as 48 carat gold simply because carat is a percentage, with 24 being pretty much 100 % purity (you cannot have 200% pure gold, it makes no sense). The most common carats used for gold in bullion, jewelery making and by goldsmiths are:

* 24 carat (millesimal fineness 999) * 22 carat (millesimal fineness 916) * 20 carat (millesimal fineness 833) * 18 carat (millesimal fineness 750) * 15 carat (millesimal fineness 625) * 14 carat (millesimal fineness 585) * 10 carat (millesimal fineness 417) * 9 carat (millesimal fineness 375) * 8 carat (millesimal fineness 333) * 1 carat (millesimal fineness 042) Millesimal fineness is a system of denoting the purity of platinum, gold and silver alloys by parts per thousand of pure metal by mass in the alloy. For example, an alloy containing 75% gold is denoted as "750".