9 (number)

9 (nine) is the natural number following 8 and preceding 10.

List of numbers — Integers

<< 0 1 2 3 4 5 6 7 8 9 >>

Cardinal nine
Ordinal ninth
Numeral system novenary
Factorization
Roman numeral IX or VIIII
Binary 1001
Hexadecimal 9

In mathematics

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. Another consequence of 9 being 10−1, is that it is also a Kaprekar number.

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 % likely to occur has an unlikelihood of 1 % or 0.01, which amounts to −log₁₀ 0.01 = 2 nines of probability. Zero probability gives zero nines (−log₁₀ 1 = 0). The purity of chemicals, the effectivity of processes, the availability of systems etc. can similarly be expressed in nines. For example, "five nines" (99.999 %) availability implies a total downtime of no more than five minutes per year.

Six recurring nines appear in the decimal places 762 through 767 of pi. This is known as the Feynman point (see also MathWorld).

In numeral systems

Base	Numeral system
2	binary	1001
3	ternary	100
4	quaternary	21
5	quinary	14
6	senary	13
7	septenary	12
8	octal	11
9	novenary	10
over 9 (decimal, hexadecimal)		9

In music

In music theory a ninth is the ninth note of a musical scale or the interval between the first note and the ninth. A ninth chord is a chord with a ninth.

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 AntonÃÂÃÂn DvořÃÂÃÂ¡k 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.

Other fields

Nine is the highest single-digit number in the decimal system. In the NATO phonetic alphabet, the digit 9 is called "niner".

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. A group of nine of anything is called an ennead.

The atomic number of fluorine
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).
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.
The name of the area called Kowloon in Hong Kong literally means: nine dragons.
In chess, the maximum number of queenss each side is allowed to have after queening all pawns.
Nine rank system
The Nine Chapters on the Mathematical Art
Nine Worthies
The album Nine by Fairport Convention
Historical years: 9 A.D., 9 B.C, or 1909
Nine ball is the standard professional billiards variant played in the United States