20 (number)

20 (twenty) is the natural number following 19 and preceding 21. A group of twenty units may also be referred to as a score.^[1]

In mathematics

20 is the basis for vigesimal number systems.
20 is the third composite number comprising the product of a squared prime and a prime, and also the second member of the (2²)q family in this form.
20 has an aliquot sum of 22 (110% in abundance). Accordingly, 20 is the third abundant number and demonstrates an 8 member aliquot sequence; {20, 22, 14, 10, 8, 7, 1, 0}.
20 is the smallest primitive abundant number.
20 is the 4th composite number in the 7-aliquot tree. Two numbers have 20 as their aliquot sum; the discrete semiprime 34 and the squared prime 361. Only 2 other square primes are abundant 12 and 18.
An icosahedron has 20 faces. A dodecahedron has 20 vertices.
20 can be written as the sum of three Fibonacci Numbers uniquely, i.e. 20 = 13 + 5 + 2.
The product of the number of divisors and the number of proper divisors of 20 is exactly 20.
20 is the number of quarter or half turns required to optimally solve a Rubik's cube in the worst case.
20 is the only number with more than one digit that can be written from base 2 to base 20 using only the digits 0 to 9.^[2]

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.

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).

The jersey number 20 has been retired by several North American sports teams in honor of past playing greats:
- In Major League Baseball:
  - The Arizona Diamondbacks, for Luis Gonzalez.
  - The Baltimore Orioles and Cincinnati Reds, both for Hall of Famer Frank Robinson.
  - The Kansas City Royals, for Frank White.
  - The Los Angeles Dodgers, for Hall of Famer Don Sutton.
  - The Philadelphia Phillies, for Hall of Famer Mike Schmidt.
  - The Pittsburgh Pirates, for Hall of Famer Pie Traynor.
  - The St. Louis Cardinals, for Hall of Famer Lou Brock.
  - The San Francisco Giants, for Hall of Famer Monte Irvin, who played for the team when it was the New York Giants.
- In the NFL:
  - The Detroit Lions, for Hall of Famer Barry Sanders.
  - The New England Patriots, for Gino Cappelletti, who played for the team when it was known by its original name of the Boston Patriots.
- In the NBA:
  - The Portland Trail Blazers, for Maurice Lucas.
- In the NHL:
  - The Los Angeles Kings, for Hall of Famer Luc Robitaille.
In the NASCAR Sprint Cup Series, car #20 is currently owned by Joe Gibbs Racing. Tony Stewart won the Cup Series title twice in the #20 car while with Joe Gibbs Racing. After Stewart left the team in 2009 to drive for himself, rookie Joey Logano took over the car.
Twenty20 is a form of limited overs cricket where each team plays only 20 overs.

20/20 is a late-night newsmagazine program on ABC-TV
In the 1974 sci-fi film Dark Star, Exponential Thermostellar Bomb number 20 threatens to detonate in the Dark Star's bomb bay
A 20-minute-long program of advertisements and trailers shown before some films playing in American movie theaters is called "The Twenty" (spelled "The 20wenty")

Twenty questions is a popular party game
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.

Bands with the number twenty in their name include Matchbox Twenty
20 (Harry Connick, Jr. album), a 1988 album by Harry Connick, Jr.
Twenty (Lynyrd Skynyrd 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.

The number of twenty can also be called a score (as Abraham Lincoln did in his Gettysburg Address or Martin Luther King in his I Have a Dream speech).
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.
In the United States Constitution, $20 is the threshold value of civil disputes above which the right to trial by jury is preserved
Yan Tan Tethera is a 20-word jingle for counting sheep
The ordinal adjective is vicenary
There are 20 ounces in Venti size coffees at Starbucks coffee shops.

20 is:

Twenty, a village in Lincolnshire
The code for international direct dial phone calls to Egypt
The designation of Interstate 20, a freeway that runs from Texas to South Carolina
CB slang for "a place", being short for "10–20", used in reference to a person or object's location
In the French and Portuguese education system, grades are given out of 20. Giving 20/20 is a very rare occasion.

^ John H. Conway and Richard K. Guy, The Book of Numbers. New York: Copernicus (1996): 11. ""Score" is related to "share" and comes from the Old Norse "skor" meaning a "notch" or "tally" on a stick used for counting. ... Often people counted in 20s; every 20th notch was larger, and so "score" also came to mean 20."
^ Sequence A131646 in The On-Line Encyclopedia of Integer Sequences