Nines are an informal, yet common method of grading the purity of very fine precious metals such as
platinum,
gold and
silver. Based on the system of
millesimal fineness, a metal is said to be
one nine or
one nine fine if it is 900 fine, or 90% pure. A metal that is 990 fine is then described as
two nines fine and one that is 999 fine is described as
three nines fine. Thus, nines are a
logarithmic scale of purity for very fine precious metals. The nines scale is also sometimes used in describing the
purity of bottled gases.