eighteen

← 17 19 → 18
← 10 11 12 13 14 15 16 17 18 19 → List of numbers — Integers 0 10 20 30 40 50 60 70 80 90 →
Cardinal	eighteen
Ordinal	18th (eighteenth)
Numeral system	octodecimal
Factorization
Divisors	1, 2, 3, 6, 9, 18
Roman numeral	XVIII
Binary	100102
Octal	228
Duodecimal	1612
Hexadecimal	1216

18 (eighteen) is the natural number following 17 and preceding 19. It equals ten and eight, twice nine, nine times two or three times six.

In speech, the numbers 18 and 80 are sometimes confused. When carefully enunciated, they differ in which syllable is stressed: 18 /eɪtˈtiːn/ vs 80 /ˈeɪti/. However, in dates such as 1864, or when contrasting numbers in the teens, such as 17, 18, 19, the stress shifts to the first syllable: 18 /ˈeɪttiːn/. In some dialects, such as General American, there is little confusion because the single t sound in 80 becomes a d-like flap [ɾ], whereas the double t sound in 18 does not.

In mathematics

Eighteen is a composite number, its proper divisors being 1, 2, 3, 6 and 9. Three of these divisors (3, 6 and 9) add up to 18, hence 18 is a semiperfect number. Eighteen is the first inverted square-prime of the form p.q².

It is a heptagonal number, and as the sum of the first three pentagonal numbers, it is a pentagonal pyramidal number. It is also the sum of the totient function for the first seven integers.

Eighteen is the second abundant number with the aliquot sum of 21 (117% in abundance) in the aliquot sequence (18,21,11,1,0) thus being the first composite number in the 11-aliquot tree. Eighteen is the aliquot sum of only one number the square 289.

In base 10 it is a Harshad number.

18, aside from 0, is the only number that equals twice the sum of its decimal digits.

Look up eighteen in Wiktionary, the free dictionary.

In science

• The atomic number of argon
• The 18-electron rule is a rule of thumb in transition metal chemistry for characterising and predicting the stability of metal complexes

In religion and literature

• The Hebrew word for "life" is חי (chai), which has a numerical value of 18. Consequently, the custom has arisen in Jewish circles to give donations and monetary gifts in multiples of 18 as an expression of blessing for long life.
• In Judaism, in the Talmud; Pirkei Avot (5:25), Rabbi Yehudah ben Teime gives the age of 18 as the appropriate age to get married ("Ben shmonah esra lechupah", at eighteen years old to the Chupah (marriage canopy)). (See Coming of age, Age of majority).
• In Ancient Roman custom the number 18 can symbolise a blood relative.
• Joseph Heller's novel Catch-22 was originally named Catch-18 because of the Hebrew meaning of the number, but was amended to the published title to avoid confusion with another war novel, Mila 18.^[1]
• There are 18 chapters in the Bhagavad Gita, which is contained in the Mahabharata, which has 18 books. The Kurukshetra War which the epic depicts, is between 18 armies (11 on the Kuru side, 7 on the Pandava).
• 18 is on the right palm of the right hand in Arabic.

As lucky or unlucky number

• In Chinese tradition, the number 18, normally 十八 (shí bā), can also be read as 幺八 (yāo bā), which sounds like 要发 (yào fā), meaning that one is going to prosper. Thus, building floors numbered "18" are often very expensive in China^{[citation needed]} The number 18 as in the floor 18 of a building in Northern China is viewed as a floor to be avoided as the 18 level of hell that one can never return from.

Age 18

The examples and perspective in this article deal primarily with North America and do not represent a worldwide view of the subject. Please improve this article and discuss the issue on the talk page.

In many parts of the world, 18 is the age minimum for purchasing tobacco and alcoholic beverages. It is also the voting age in many countries. During the Vietnam War, Americans could be drafted at an age as low as 18, but were not allowed to vote in most states until they were 21. A movement spread to lower the voting age, and in 1971, the Twenty-sixth Amendment extended a guarantee of suffrage to anyone 18 years or older in all states. The voting age was lowered from 21 to 18 in the U.K. and many other nations around that time.

• Eighteen is also the age in many countries at which a person may appear in a pornographic video and in some countries express legal consent to sexual relations with another.^{[citation needed]}
• In addition, eighteen-year-olds in the United States have the right to bear arms, own property, marry without parental consent (in almost all states), get an abortion, donate their bodies to science, and serve on a jury.
• In the United Kingdom, eighteen-year-olds can purchase tobacco, alcohol and pornography, go to war, vote, and model for pornography; (but the age of consent is 16).
• In most European countries 18 is the legal driving age without a parent present.
• In many countries 18 is the age of majority.
• In many states the age at which one can obtain a learner's permit without parental consent.
• Minimum age at which one can be admitted to rent or buy an MPAA-rated NC-17 movie. This is also the age at which one can see an NC-17 rated movie.^{[citation needed]}
• Minimum age at which one can rent or buy an ESRB-rated AO game.
• Minimum age at which one can buy tickets to an 18A rated film in Canada without being accompanied by an adult. Ratings in Canada are provincial, so they may vary. A film can be 18A in some provinces and 14A in other provinces. A film can also be 18A in some provinces and R in other provinces. 18 is also the minimum age at which one can view, rent, or buy an R or an A rated movie.
• Minimum age at which one can buy, rent, view, or buy tickets to an 18+ rated movie in the Canadian province of Quebec.
• Minimum age at which one can get a full driver's license in many U.S states and Canadian provinces.
• Minimum age to drive a car in many countries in the world.

See also: 18 certificate, Coming of age

• In the United States, it is the minimum age at which one may get a body piercing without a parent present.
• In the United States, it is the minimum age at which one may get a tattoo without parental consent.
• It is the legal age at which one may gamble in some states in the United States.

In other fields

Eighteen is also:

• a 2005 movie directed by Richard Bell and starring Brendan Fletcher and Alan Cumming.
• Bobby Labonte's car number in the NASCAR Winston Cup Series when he won the 2000 Championship. This is J. J. Yeley's car number until the end of the 2007 Nextel Cup Season. Kyle Busch took the car number in 2008.
• A 2002 album by electronic artist Moby.
• The number of holes in a stipulated round of golf
• The number of chapters into which James Joyce's epic novel Ulysses is divided
• Russ Wheeler's car number in the film Days of Thunder
• A slang term used in association football for the Penalty Area
• The number of wheels on the most common type of North American tractor-trailer truck, which are hence often called 18-wheelers
• The customary percentage to tip the serving staff in a restaurant by groups of four or more^{[citation needed]}
• The number of the French department Cher and the Turkish province Bitlis
• DVD-18 is double-sided, double-layered DVD format
• A Canon error message, see E18 error
• In neo-Nazi circles, a code word for Adolf Hitler. The number comes from the position of the letters in the alphabet: A = 1, H = 8. See also 88
• Android 18, a fictional character in the metaseries Dragon Ball.
• 18th Street gang, a Hispanic American gang
• The Hindu epic Mahabharata has eighteen sections, involves eighteen armies and is about a war fought over eighteen days
• In some countries the number 18 means blood (relative)
• In the Chinese mythos, Hell has 18 levels
• In Chinese folklore, the Shaolin temple had a group of 18 bronze Monks. Initiates could only be considered full graduates of Shaolin martial arts if they could defeat them in combat.
• In 1990 Megadeth released Hanger 18 as a single for the Rust in Piece Album.
• Eighteen Visions (often abbreviated to 18V), an alternative metal/metalcore band from Orange County, California.
• Eighteen Eighteen, a South African Rap artist.
• "I'm Eighteen", Alice Cooper's first Top Ten hit single, from their 1971 album Love It to Death.

Historical years

• A.D. 18
• 18 B.C.
• 1918
• 2018

See also

• Numeral system

Notes