Nine

Nine is a composite number, its proper divisors being 1 and 3.

In base-10 a number is evenly divisible by nine if and only ifthe iterative sum of it's digits reduces to 9. This is equivalent to saying a number is divisible by 9 if and only if its decimal digit total is divisible by 9. The only other number with this property is three.

In probability, the nine is a logarithmic measure of probability of an event, defined as the negative of the base-10 logarithm of the probability of the event's complement. For example, an event that is 99 percent likely to occur has 0.01 (1 percent) unlikelihood, or -log₁₀(0.01) = 2 nines of probability. Zero probability gives zero nines (-log₁₀(1) = 0).

Availability of a system, which refers to the probability that a system is available at any given moment, is often measured in "nines". For example, a system that is 99.9% reliable would be referred to as having a reliability of "three nines".

Many computer and communications facilities strive toward "five nines" (99.999 percent) availability, which implies a total downtime[?] of no longer than five minutes per year.

The effectiveness of spam filters and the purity of materials are also often stated in nines.

Nine is considered an unlucky number in Japanese culture.

Nine (九 pinyin jiu3) is considered a good number in Chinese culture because it sounds the same as the word "longlasting" (久 pinyin jiu3).

A polygon with nine sides is a nonagon[?].

In the NATO phonetic alphabet, the digit 9 is called "niner".

Nine is also:

• the number of musicians in a nonet[?].

This article is about the number. For the year, see 9.