Nine (purity)
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. Similarly, percentages ending in a 5 have conventional names, traditionally the number of nines, then "five", so 999.5 fine (99.95% pure) is "three nines five", abbreviated 3N5.[1][2] 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.