21 (number)

21 (twenty-one) is the natural number following 20 and preceding 22.


Cardinal	21 twenty-one
Ordinal	21st twenty-first
Factorization	$3 \cdot 7$
Roman numeral	XXI
Binary	10101
Hexadecimal	15

In mathematics

Twenty-one is a Fibonacci number, a Harshad number, a Motzkin number, a triangular number and an octagonal number, as well as a composite number, its proper divisors being 1, 3 and 7.

21 appears in the Padovan sequence, preceded by the terms 9, 12, 16 (it is the sum of the first two of these).

Adding up the sums of divisors for the integers 1 through 5 yields 21.

21 is a repdigit in base 4 (111).

In science

The atomic number of scandium

Astronomy

Messier object M21, a magnitude 7.0 open cluster in the constellation Sagittarius

The New General Catalogue object NGC 21, a barred spiral galaxy in the constellation Andromeda

The Saros number of the solar eclipse series which began on -2275 May 5 and ended on -995 June 11. The duration of Saros series 21 was 1280.1 years, and it contained 72 solar eclipses.

The Saros number of the lunar eclipse series which began on -1973 February 11 and ended on -639 April 22. The duration of Saros series 21 was 1334.2 years, and it contained 75 lunar eclipses.

Age 21

In the United States (excluding insular areas), 21 is the minimum legal age at which one can purchase and drink alcoholic beverages. It is also the age at which it is legal to gamble and work as a prostitute in Nevada (except in Storey County and Lyon County, where the legal age is 18). In most states, a person who is 21 or over may also rent a car. Twenty-one was the minimum age of suffrage in the majority of states until the introduction of the Twenty-sixth Amendment.

Twenty-one was the age of majority in England and Wales before that age was lowered to 18 with the enactment of the Family Law Reform Act on 1 January 1970.

In sports

Players with jersey number twenty-one include:

Mike Eruzione of the 1980 Men's Gold Olympic Hockey Team
Roberto Clemente, Hall-of-Famer of the Pittsburgh Pirates
Peter Forsberg, hockey player for the Quebec Nordiques, Colorado Avalanche and now Philadelphia Flyers

21 is a variation of street basketball, in which each player, of which there can be any number, plays for himself only (i.e. not part of a team); the name comes from the requisite number of baskets.

In other fields

21 is:

The current century—spanning the years from 2001 to 2100—referred to as the 21st century. The approach of this century inspired many forward-looking 20th century events and company and product names, such as the Century 21 Exposition (another name for the 1962 Seattle World's Fair), Century 21 Real Estate, and Century 21 Television (producers of Sylvia and Gerry Anderson Supermarionation shows like Thunderbirds and Captain Scarlet).
The Twenty-first Amendment repealed the Eighteenth Amendment, thereby ending Prohibition.
Pier21 was, from 1928 to 1971, the place where immigrants entered Canada. It was called the "Gateway to Canada."
Forever 21 (also operating as XXI Forever) is a U.S.-based chain of clothing stores that was founded in 1984.
The number of spots on a standard cubical dice (1+2+3+4+5+6)
The number of firings in a 21-gun salute honoring Royalty or leaders of countries
A card game, also called vingt-et-un (French for "twenty-one"), pontoon, or blackjack
A drinking game in which the goal is to get the group to 21 without saying it oneself
The title of a TV quiz show that ran from 1956 to 1958, most remembered for the scandal that the wins were fixed (see Quiz show scandals). It was remade in 2000
The title of at least three different movies:

A 1918 comedy starring Bryant Washburn and Gertrude Selby

A 1923 drama starring Richard Barthelmess and Dorothy Mackaill

A 1991 drama starring Patsy Kensit

In the title of the TV show 21 Jump Street, starring Johnny Depp
The standard TCP/IP port number for FTP connection
Twenty-One Demands were a set of demands which were sent to the Chinese government by the Japanese government of Okuma Shigenobu in 1915
Number 21 is the name of the plane alleged flown by Gustave Whitehead two years before the Wright brothers' flight
The title of a song by The Cranberries
21 is the title of a novel by Patrick O'Brian
The car number of NASCAR race cars owned by the Wood Brothers
The 21 Club is a top restaurant in New York City
In Israel, the number is associated with the profile 21 (the military profile designation granting an exemption from the military service)
21 grams is the weight of the soul, according to research by Duncan MacDougall, generally regarded as meaningless. In reference to MacDougall's theory, 21 Grams is the title of a movie (although MacDougall's theory plays no direct role in the film)
The British version of the novel A Clockwork Orange ended on chapter 21

Historical years

21 A.D., 21 B.C., 1921, 2021, etc.

Integers