12 (number)
From Academic Kids

Cardinal  12 twelve 
Ordinal  12th twelfth 
Numeral system  duodecimal 
Factorization  <math>2^2 \cdot 3<math> 
Divisors  1, 2, 3, 4, 6, 12 
Roman numeral  XII 
Unicode representation of Roman numeral  Ⅻ, ⅻ 
Binary  1100 
Octal  14 
Duodecimal  10 
Hexadecimal  C 
Mathematical properties  
φ(12) = 4  τ(12) = 6 
σ(12) = 28  π(12) = 5 
μ(12) = 0  M(12) = 2 
12 (twelve) is the natural number following 11 and preceding 13.
Contents 
In mathematics
Twelve is a composite number, its proper divisors being 1, 2, 3, 4 and 6. Twelve is also a highly composite number, the next one being 24.
Twelve is a superfactorial, being the product of the first three factorials.
Twelve being the product of three and four, the first four positive integers show up in the equation 12 = 3 × 4, which can be continued with the equation 56 = 7 × 8.
A twelvesided polygon is a dodecagon. A twelvesided polyhedron is a dodecahedron. Twelve is a pentagonal number.
The number twelve is often used as a sales unit in trade, and is often referred to as a dozen. Twelve dozen are known as a gross. (Note that there are thirteen loaves in a baker's dozen.) The duodecimal system (12_{10} [twelve] = 10_{12}), which is the use of 12 as a division factor for many ancient and medieval weights and measures, including hours, probably originates from Mesopotamia. See also: base 12. As Schoolhouse Rock explains in its song "Little Twelvetoes", if humankind had been born with twelve fingers, they would have counted and multiplied using the duodecimal system. (There is no need to have twelve fingers though, since one will easily arrive at the duodecimal system by simply counting the phalanx of fingers with the same hand's thumb, a practice in use with some people until the present day.)
In base thirteen and higher bases (such as hexadecimal), twelve is represented as C.
Etymology (Weekley, Skeat) suggests that "twelve" (similar to "eleven") consists of two parts, the first meaning "two" and the second "leftover", so a literal translation would yield "two remaining [after having ten taken]".
Twelve is superabundant, sparsely totient, a Harshad number and a Pell number.
In science
The atomic number of magnesium
In astronomy,
 Messier object M12, a magnitude 8.0 globular cluster in the constellation Ophiuchus
 The New General Catalogue object (http://www.ngcic.org/) NGC 12, a magnitude 13.1 spiral galaxy in the constellation Pisces
 The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1175.html) of the solar eclipse series which began on 2680 August 8 and ended on 1129 February 25. The duration of Saros series 12 was 1550.5 years, and it contained 87 solar eclipses.
 The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1175.html) of the lunar eclipse series which began on 2360 May 28 and ended on 1062 July 17. The duration of Saros series 12 was 1298.1 years, and it contained 73 lunar eclipses.
In time
There are twelve months in a year.
The Western zodiac has twelve signs, as does the Chinese zodiac.
There are twentyfour hours in a day in all, with twelve hours for a half a day. The hours are numbered from one to twelve for both the ante meridiem (a.m.) half of the day and the post meridiem (p.m.) half of the day. 12:00 after a.m. and before p.m. (in the middle of the day) is noon, and 12:00 after p.m. and before a.m. (in the middle of the night) is midnight. A new day is considered to start with the stroke of midnight.
The minute hand of a clock face turns twelve times faster than the hour hand.
In other fields
In English, twelve is the number of greatest magnitude that has just one syllable.
There are normally twelve pairs of ribs in the human body.
Jewish tradition and the old testament of the Bible talks of twelve tribes of Israel. The New Testament describes twelve apostles of Jesus. The Book of Revelation 12:1 mentions a woman  usually interpreted as Virgin Mary  wearing a crown of twelve stars.
Twelve stars are also featured on the European flag.
In modern Jewish practice, a girl gets her bat mitzvah on the Shabbat after her twelfth birthday.
In the United States, twelve people are appointed to sit on a jury for felony trials in all but four states, and in federal and D.C. courts. The number of jurors in the movie gave the title to the film 12 Angry Men.
In Astrology, Pisces is the 12th and final astrological sign of the Zodiac.
Other movies with the number twelve in their titles include The Dirty Dozen, 12 Monkeys, Ocean's Twelve and Twelve Chairs.
The song Twelve Days of Christmas came from the traditional practice of extending Yuletide celebrations over the twelve days from Christmas day to the eve of Epiphany; the period of thirteen days including Epiphany is sometimes known as as Christmastide.
Twelfth Night, another name for the 12th Day of Christmas or January 5 (the eve of Epiphany), gives its name to a Shakespeare comedy, as well as that of progressive rock band.
Twelve is:
 ASCII and Unicode code point for form feed.
 In music, the number of pitch classes in an octave, not counting the duplicated (octave) pitch. Also, the total number of major keys, (not counting enharmonic equivalents) and the total number of minor keys (also not counting equivalents). This applies only to twelve tone equal temperament, the most common tuning used today in western influenced music.
 In Greek mythology, the number of labours of Heracles (Hercules).
 The number of dan (master) grades in judo.
 The designation of United States Interstate 12, a freeway in Louisiana.
 In the Netherlands, highway A12 connects The Hague via Utrecht and Arnhem with the German border. In Germany, highway A12 connects Berlin with the Polish border near Frankfurt an der Oder. In Belgium, highway A12 connects Brussels via Antwerp with the Netherlands
 European route E12  Mo i Rana  Umeå ... Vaasa  Tampere  Hämeenlinna  Helsinki
 In rugby union one of the centres, most often but not always the inside centre, wears the 12 shirt.
 The number of function keys on most PC keyboards (F1, F2, F3, F4, F5, F6, F7, F8, F9, F10, F11 and F12)
 The number of dialling keys in any standard digital telephone (1, 2, 3, 4, 5, 6, 7, 8, 9, 0, * and #)
 In the Ishihara color test, 12 is on the first plate which both persons with normal vision and colorblind persons can discern
 Historical years: A.D. 12, 12 B.C., or 1912
External links
Why twelve (http://koenraadelst.voiceofdharma.com/articles/misc/whytweve.html) Article on the number 12da:12 (tal) de:Zwölf es:Doce eo:Dek du fr:12 (nombre) ko:12 ia:12 it:Dodici nah:Mahtlahtlinome nl:Twaalf ja:12 no:12 (tall) nn:Talet 12 ru:12 (число) sl:12 (število) sv:12 (tal) zh:12