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Nine (9) is the natural number following eight and preceding ten. It is the highest single-digit number in the decimal system.

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 if the iterative sum of its 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 base-N, the divisors of N-1 have this property.

Six recurring nines appear in the decimal places 762 through 767 of pi.

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.

Someone dressed "to the nines" is dressed up as much as they can be.

Stanines are measured on a scale of 1 to 9.

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).

Nine is the number of musicians in a nonet. Nine babies born into a single birth are called nonuplets, although not one baby born into a set of nonuplets has ever survived infancy. A polygon with nine sides is an enneagon or nonagon. A group of nine of anything is called an ennead.

In binary code nine is 1001; in ternary code nine is 100; in quaternary numeral system code nine is 21; in quinary nine is 14; in senary nine is 13; in septenary nine is 12; in octal nine is 11; in novenary nine is 10; in decimal code and above (for instance hexadecimal) nine is 9. In Roman numerals nine is IX.

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

There are nine planets in our solar system, each with its mythological and astrological significance, although three of these (Uranus, Neptune, Pluto) were not known as planets in ancient times.

There are nine basic personality types represented on the enneagram.

In Greek mythology there are nine Muses.

Nine judges sit on the United States Supreme Court.

It takes nine to make a baseball team.

Nine Lives Cat Food got its name from the legend that a cat has nine lives.

A novena lasts for nine days.

Nine months is the gestation period for humans.

Joel Garreau identified nine different "nations" of North America.

A standard work day of 9 to 5 begins at 9 a.m.

Bands with the number nine in their name include Stroke 9, Nine Days and Nine Inch Nails. There is also a song called "Love Potion #9".

Both Ludwig van Beethoven and Anton Bruckner wrote nine symphonies. After Beethoven died leaving his Tenth Symphony unfinished, composers were superstitious about writing Ninth Symphonies for the rest of the nineteenth century. Gustav Mahler tried to "cheat death" by writing his Ninth Symphony but calling it Das Lied von der Erde instead. Afterwards he wrote a work called Ninth Symphony, but thinking of it as his Tenth. He died with his Tenth (or Eleventh in his mind) unfinished. Perhaps Antonín Dvořák was also superstitious about the number nine, because he wrote no symphonies after his New World Symphony, which is nowadays considered his Ninth, but which he thought was his Eighth because he considered the score of his early C minor Symphony lost forever. He lived for seven more years. In the twentieth century, a handful of composers, such as Dmitri Shostokovich, have written a Ninth Symphony and lived to write more.

The name of the area called Kowloon in Hong Kong literally means: nine dragons.

Other uses include:

See also: seven, eight, nine, ten, eleven, integer, list of numbers.

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

Nine is an album by Fairport Convention