# 18 (number)

(Redirected from XVIII)
 ← 17 18 19 →
Cardinal eighteen
Ordinal 18th
(eighteenth)
Numeral system octodecimal
Factorization 2 × 32
Divisors 1, 2, 3, 6, 9, 18
Greek numeral ΙΗ´
Roman numeral XVIII
Binary 100102
Ternary 2003
Quaternary 1024
Quinary 335
Senary 306
Octal 228
Duodecimal 1612
Vigesimal I20
Base 36 I36

18 (eighteen) is the natural number following 17 and preceding 19.

In English speech, the numbers 18 and 80 are sometimes confused, as they sound very similar.

## In mathematics

Eighteen is a composite number, its 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.[1] Eighteen is the first inverted square-prime of the form p·q2.

## 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).
• Shemoneh Esrei (sh'MOH-nuh ES-ray) is a prayer that is the center of any Jewish religious service. Its name means "eighteen". The prayer is also known as the Amidah.
• 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.[2]
• 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). The war itself lasts for 18 days. In the other Hindu epic, the Ramayana, the war between Rama and the demons also lasted 18 days.

## As lucky or unlucky number

• In Chinese tradition, 18 pronounced 十八 (shí bā) and is a considered lucky number due to similarity with 實發 (shì fā) 'definitely get rich', 'to get rich for sure'.[3]

## Age 18

In most countries, 18 is the age of majority and also the voting age.

In eleven states of the United States, 18 is the age of sexual consent.

It is also the age of sexual consent under USA federal law (although this law only applies in circumstances involving transportation or transaction between different states or outside the USA).[4]

In addition, Pennsylvania has a corruption of minors law that makes the de facto age of consent there 18.[5] This is also the case in some jurisdictions outside the USA,[6] including Canada.[7][8]

## In other fields

Eighteen is also: