20 (twenty; Roman numeral XX) is the

natural number In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country"). Numbers used for counting are called ''cardinal ...

following 19 and preceding 21. A group of twenty units may also be referred to as a score.

In mathematics

*20 is a

pronic number A pronic number is a number that is the product of two consecutive integers, that is, a number of the form n(n+1).. The study of these numbers dates back to Aristotle. They are also called oblong numbers, heteromecic numbers,. or rectangular number ...

. *20 is a

tetrahedral number A tetrahedral number, or triangular pyramidal number, is a figurate number that represents a pyramid with a triangular base and three sides, called a tetrahedron. The th tetrahedral number, , is the sum of the first triangular numbers, that is, ...

as 1, 4, 10, 20. *20 is the basis for vigesimal number systems. *20 is the third

composite number A composite number is a positive integer that can be formed by multiplying two smaller positive integers. Equivalently, it is a positive integer that has at least one divisor other than 1 and itself. Every positive integer is composite, prime, ...

to be the product of a squared prime and a prime, and also the second member of the (''2''²)''q'' family in this form. *20 is the smallest primitive abundant number. *An icosahedron has 20 faces. A

dodecahedron In geometry, a dodecahedron (Greek , from ''dōdeka'' "twelve" + ''hédra'' "base", "seat" or "face") or duodecahedron is any polyhedron with twelve flat faces. The most familiar dodecahedron is the regular dodecahedron with regular pentagon ...

has 20 vertices. *20 can be written as the sum of three

Fibonacci number In mathematics, the Fibonacci numbers, commonly denoted , form a sequence, the Fibonacci sequence, in which each number is the sum of the two preceding ones. The sequence commonly starts from 0 and 1, although some authors start the sequence from ...

s uniquely, i.e. 20 = 13 + 5 + 2. *20 is the number of moves (quarter or half turns) required to optimally solve a Rubik's Cube in the worst case."God's Number is 20"
Cube20.org *20 is the number of parallelogram polyominoes with 5 cells. *20 is a Harshad number

In science

*The

atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

. *The third magic number in physics. *The

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

shower number for Coma Berenicids.

Biology

*The number of

proteinogenic Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation. The word "proteinogenic" means "protein creating". Throughout known life, there are 22 genetically encoded (proteinogenic) amino aci ...

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...

s that are encoded by the standard genetic code. *In some countries, the number 20 is used as an index in measuring

visual acuity Visual acuity (VA) commonly refers to the clarity of vision, but technically rates an examinee's ability to recognize small details with precision. Visual acuity is dependent on optical and neural factors, i.e. (1) the sharpness of the retinal ...

. 20/20 indicates normal vision at 20 feet, although it is commonly used to mean "perfect vision". (Note that this applies only to countries using the Imperial system. The metric equivalent is 6/6.) When someone is able to see only after an event how things turned out, that person is often said to have had "20/20 hindsight".

As an indefinite number

* A 'score' is a group of 20 (often used in combination with a

cardinal number In mathematics, cardinal numbers, or cardinals for short, are a generalization of the natural numbers used to measure the cardinality (size) of sets. The cardinality of a finite set is a natural number: the number of elements in the set. T ...

, i.e. ''fourscore'' to mean 80), but also often used as an indefinite number"Biblical Criticism", ''The Classical Journal'' 36:71:83''ff'' (March 1827
full text
/ref> (e.g. the newspaper headline "Scores of Typhoon Survivors Flown to Manila")."CBS News"
''Scores of Typhoon Survivors Flown to Manila''
(November 2013)

In sports

Twenty20 Twenty20 (T20) is a shortened game format of cricket. At the professional level, it was introduced by the England and Wales Cricket Board (ECB) in 2003 for the inter-county competition. In a Twenty20 game, the two teams have a single inn ...

is a form of limited overs cricket where each team plays only 20 overs. * A standard

dartboard Darts or dart-throwing is a competitive sport in which two or more players bare-handedly throw small sharp-pointed missiles known as darts at a round target known as a dartboard. Points can be scored by hitting specific marked areas of the bo ...

is laid out as 20

sectors Sector may refer to: Places * Sector, West Virginia, U.S. Geometry * Circular sector, the portion of a disc enclosed by two radii and a circular arc * Hyperbolic sector, a region enclosed by two radii and a hyperbolic arc * Spherical sector, a p ...

. * The Kentucky Derby currently has a maximum field of 20 horses. * In

rugby union Rugby union, commonly known simply as rugby, is a close-contact team sport that originated at Rugby School in the first half of the 19th century. One of the two codes of rugby football, it is based on running with the ball in hand. In it ...

, 20 national teams currently qualify for each edition of the men's

Rugby World Cup The Rugby World Cup is a men's rugby union tournament contested every four years between the top international teams. The tournament is administered by World Rugby, the sport's international governing body. The winners are awarded the Webb E ...

. * In

chess Chess is a board game for two players, called White and Black, each controlling an army of chess pieces in their color, with the objective to checkmate the opponent's king. It is sometimes called international chess or Western chess to dist ...

, 20 is the number of legal moves for each player in the starting position. * In Australian rules football, most games consist of four 20-minute quarters. * Under NCAA men's basketball rules, games consist of 20-minute halves. *

Ice hockey Ice hockey (or simply hockey) is a team sport played on ice skates, usually on an ice skating rink with lines and markings specific to the sport. It belongs to a family of sports called hockey. In ice hockey, two opposing teams use ice h ...

games are played in three 20-minute periods.

Age 20

*In many disciplines of developmental psychology, adulthood starts at age 20. *Formerly the

age of majority The age of majority is the threshold of legal adulthood as recognized or declared in law. It is the moment when minors cease to be considered such and assume legal control over their persons, actions, and decisions, thus terminating the contr ...

in Japan and in Japanese tradition. *Age 20 is the age at which

Levites Levites (or Levi) (, he, ''Lǝvīyyīm'') are Jewish males who claim patrilineal descent from the Tribe of Levi. The Tribe of Levi descended from Levi, the third son of Jacob and Leah. The surname ''Halevi'', which consists of the Hebrew de ...

in the time of King David were allowed "to do the work for the service of the house of the Lord", the

Temple in Jerusalem The Temple in Jerusalem, or alternatively the Holy Temple (; , ), refers to the two now-destroyed religious structures that served as the central places of worship for Israelites and Jews on the modern-day Temple Mount in the Old City of Jeru ...

(see First Chronicles Chapter 23, verses 24 and 27). In the time of

Ezra Ezra (; he, עֶזְרָא, '; fl. 480–440 BCE), also called Ezra the Scribe (, ') and Ezra the Priest in the Book of Ezra, was a Jewish scribe ('' sofer'') and priest (''kohen''). In Greco-Latin Ezra is called Esdras ( grc-gre, Ἔσδρα ...

and

Nehemiah Nehemiah is the central figure of the Book of Nehemiah, which describes his work in rebuilding Jerusalem during the Second Temple period. He was governor of Persian Judea under Artaxerxes I of Persia (465–424 BC). The name is pronounced o ...

, following the Babylonian captivity, it was Levites from the age of 20 upwards who were assigned "to oversee the work of the house of the LORD" (

Chapter 3, verse 8).

Music

In other fields

* +20 is the

code for international direct dial Country calling codes or country dial-in codes are telephone number prefixes for reaching telephone subscribers in the networks of the member countries or regions of the International Telecommunication Union (ITU). The codes are defined by the ...

phone calls to

Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Medit ...

. *

CB slang CB slang is the distinctive anti-language, argot or cant which developed among users of Citizens Band radio (CB), especially truck drivers in the United States during the 1970s and early 1980s. The slang itself is not only cyclical, but also ...

for a place, being short for the

ten-code Ten-codes, officially known as ten signals, are brevity codes used to represent common phrases in voice communication, particularly by law enforcement and in Citizens Band (CB) radio transmissions. The police version of ten-codes is officially k ...

"10–20" meaning "What is your location?" *'' 20/20'' is a primetime newsmagazine program on ABC, which in turn is taken from the expression for normal eyesight (

20/20 vision Visual acuity (VA) commonly refers to the clarity of vision, but technically rates an examinee's ability to recognize small details with precision. Visual acuity is dependent on optical and neural factors, i.e. (1) the sharpness of the retinal ...

). *''020'' is the

ISO 3166-1 numeric ISO 3166-1 numeric (or numeric-3) codes are three-digit country codes defined in ISO 3166-1, part of the ISO 3166 standard published by the International Organization for Standardization (ISO), to represent countries, dependent territories, and sp ...

3 digit

country code Country codes are short alphabetic or numeric geographical codes (geocodes) developed to represent countries and dependent areas, for use in data processing and communications. Several different systems have been developed to do this. The term ...

for

Andorra , image_flag = Flag of Andorra.svg , image_coat = Coat of arms of Andorra.svg , symbol_type = Coat of arms , national_motto = la, Virtus Unita Fortior, label=none (Latin)"United virtue is stro ...

. *The

UIC Country Code The UIC Country Code is a two digit-number identifying countries in which members of the International Union of Railways (UIC) are active. The UIC has issued numbering systems for rolling stock ( UIC wagon numbers) and stations that include the cou ...

for

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eig ...

identifying member countries of the International Union of Railways (UIC). *20 is the value of the letter '' J'' when computing the

check digit A check digit is a form of redundancy check used for error detection on identification numbers, such as bank account numbers, which are used in an application where they will at least sometimes be input manually. It is analogous to a binary parity ...

in the serial number of an intermodal (shipping) container, as defined by

ISO 6346 ISO 6346 is an international standard covering the coding, identification and marking of intermodal (shipping) containers used within containerized intermodal freight transport by the International Organization for Standardization (ISO). The ...

. *In Hebrew numerals, the letter Kaph ( כ) represents twenty; see also

gematria Gematria (; he, גמטריא or gimatria , plural or , ''gimatriot'') is the practice of assigning a numerical value to a name, word or phrase according to an alphanumerical cipher. A single word can yield several values depending on the cipher ...

, the Hebrew system of numerology. Similarly the

Arabic Arabic (, ' ; , ' or ) is a Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C. E.Watson; Walter ...

letter

kāf Kaph (also spelled kaf) is the eleventh letter of the Semitic abjads, including Phoenician kāp , Hebrew kāf , Aramaic kāp , Syriac kāp̄ , and Arabic kāf (in abjadi order). The Phoenician letter gave rise to the Greek kappa (Κ), Lati ...

( ك) represents twenty in the

Abjad numerals The Abjad numerals, also called Hisab al-Jummal ( ar, حِسَاب ٱلْجُمَّل, ), are a decimal alphabetic numeral system/ alphanumeric code, in which the 28 letters of the Arabic alphabet are assigned numerical values. They have been ...

. *The number of questions in the game Twenty Questions.

References

External links

* {{Integers, zero Integers