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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.
Nines are used in a similar manner to describe computer system availability.