"Twenty" redirects here. For the village in England, see Twenty, Lincolnshire.

20 (twenty) is the natural number following 19 and preceding 21. A group of twenty units may also be referred to as a score.

In mathematics

Twenty is a composite number, its proper divisors being 1, 2, 4, 5 and 10. Skipping 2 and adding the other divisors gives 20, hence 20 is a semiperfect number, and since it is not divisible by any of the smaller semiperfect numbers, it is a primitive semiperfect number. 20 is the smallest abundant number that is not a multiple of 3.20 is also a composite number.

20 is the second positive integer with a prime factorization of the form $2^2\; cdot\; q,$ where q is a prime. Twenty is the third number in abundance (110%) with the aliquot sum of 22 within the 8-member aliquot sequence (20,22,14,10,8,7,1,0). Twenty is the aliquot sum of the discrete biprime, 34 and the square 361.

It is also the sum of the first four triangular numbers, making it a tetrahedral number.

20 is the total number of digits on the human body, which makes it a convenient number system radix; such number systems are called vigesimal. In the former British currency system, there were twenty shillings in a pound.

A polyhedron with twenty faces is an icosahedron.

Twenty is the first number in an early Indian magic square of order 3.

In base 10, it is a Harshad number and the smallest even self number.

The Monster group contains 20 sporadic groups.

In science

• The atomic number of calcium.
• The third Magic number (physics).

Biology

• The number of proteinogenic amino acids that are encoded by the standard genetic code.
• In some countries, the number 20 is used as an index in measuring visual acuity. 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".
• There are 20 baby teeth in the deciduous dentition.

In religion

• Age 20 is the age at which Levites 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 (see First Chronicles Chapter 24, verses 24 and 27). In the time of Ezra and Nehemiah, 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" (Ezra Chapter 3, verse 8).

In sports

• 20 is the retired number of Pro Football Hall of Fame running back Barry Sanders, formerly of the Detroit Lions.
• The retired number of former baseball stars Frank Robinson, Mike Schmidt, Mark Fidrych and Lou Brock.
• 20 is the number of Tony Stewart's 2-time winning NASCAR NEXTEL Cup Series car.
• The Twenty20 Cup is a form of limited Overs Cricket where each team plays only 20 Overs.
• The number of NFL stars Ed Reed, Ronde Barber and Brian Dawkins.
• 20 is the number worn by NBA player Manu Ginobili of the San Antonio Spurs.
• 20 is the number worn by NBA point guard Gary Payton formerly of Seattle SuperSonics
• 20 is the iconic number worn by football player Ole Gunnar Solskjær of Manchester United.
• 20 is the number worn by New York Yankees all-star catcher Jorge Posada.
• A twenty-game winner (pitcher) in baseball is acclaimed as having reached a milestone in a season. The most remarkable of 20-game winners was probably Hank Borowy, who won 10 games with the Yankees in 1945 before being put on waivers, then signed with the Cubs and won 11 more games playing the rest of the season. He had been in the World Series with the Yanks and played for Chicago in the '45 Series as well.
• Lukas Podolski, German National soccer player, wears number 20.
• Polish soccer player Zbigniew Boniek wore number 20 in the National Team.
• 20 is worn by NHL all-star goaltender Evgeni Nabokov of the San Jose Sharks.
• 20 is worn by the MLS Star Taylor Twellman of the New England Revolution.
• 20 is worn by NBA superstar Ray Allen, who currently plays for the Boston Celtics.
• 20 is worn by Portugal's midfielder Deco.

In other fields

• The number of twenty can also be called a score (as Abraham Lincoln did in his Gettysburg Address).
• In a game of chess both players have from which to choose 20 first moves
• Twenty questions is a popular party game
• Twenty is the age of majority in Japanese tradition. Someone who is exactly twenty years old is described as hatachi
• The Twenty Year Curse refers to the pattern of presidents of the United States who were elected to office in 1840, 1860, 1880, 1900, 1920, 1940, and 1960 to die in office. This pattern ended with the 1980 presidency of Reagan, who survived his time in office and, notably, an attempted assassination.
• Bands with the number twenty in their name include Matchbox Twenty
• A 20-minute-long entertainment program shown before some films playing in United Artists movie theaters is called "The Twenty" (spelled "The 20wenty")
• Yan Tan Tethera is a 20-word jingle for counting sheep
• The ordinal adjective is vicenary
• 20/20 is a late-night newsmagazine program on the ABC network, that has been hosted by Barbara Walters, Hugh Downs, Elizabeth Vargas, and others
• In the roleplaying game Dungeons and Dragons (as well as other RPGs that use twenty-sided dice), twenty-sided dice play a pivotal role in gameplay, and to "roll a twenty" is significant to the point that it is sometimes used in other, usually related, contexts, similar to the use of "doubles" in reference to Monopoly.
• Cigarettes are usually packaged with 20 in each pack.

20 is:

• Twenty, a village in Lincolnshire
• In the United States Constitution, $20 is the threshold value of civil disputes above which the right to trial by jury is preserved
• A denomination of U.S. dollar featuring Andrew Jackson's portrait
• A denomination of Pound sterling featuring Adam Smith's portrait
• The code for international direct dial phone calls to Egypt
• The designation of Interstate 20, a freeway that runs from Texas to South Carolina
• 20 (album), a 1988 album by Harry Connick, Jr.
• Twenty (album), a 1997 album by Lynyrd Skynyrd
• 20 (Terminaator album), a 2007 album by Terminaator
• Twenty (concert), a 2006 concert celebrating the 20th Anniversary of Regine Velasquez held at the Araneta Coliseum, Philippines. It was awarded Best Major Concert Act by the Aliw Awards.
• CB slang for "a place", being short for "10-20", used in reference to a person or object's location
• One of the TCP/IP ports used for File Transfer Protocol

Historical years

20 A.D., 20 B.C., 1920, 2020, etc.

References