7 (seven) is the natural number following 6 and preceding 8.
The SI prefix for 10007 is zetta (Z), and for its reciprocal zepto (z).
Evolution of the glyph
In the beginning, various Indians wrote 7 more or less in one stroke as a curve that looks like an uppercase J upside down. The western Ghubar Arabs' main contribution was to make the longer line diagonal rather than straight, though they showed some tendencies to making the character more rectilinear. The eastern Arabs developed the character from a 6-look-alike into an uppercase V-look-alike. Both modern Arab forms influenced the European form, a two-stroke character consisting of a horizontal upper line joined at its right to a line going down to the bottom left corner, a line that is slightly curved in some font variants. As is the case with the European glyph, the Cham and Khmer glyph for 7 also evolved to look like their glyph for 1, though in a different way, and so they were also concerned with making their 7 more different. For the Khmer this often involved adding a horizontal line above the glyph. This is analogous to the horizontal stroke through the middle that is sometimes used in handwriting in the Western world but which is almost never used in computer fonts.
On the seven-segment displays of pocket calculators and digital watches, 7 is the number with the most common glyph variation (0 and 6 also have variant glyphs). Most calculators use three line segments, but on Sharp, Casio and a few other brands of calculators, 7 is written with four line segments.
Seven is the fourth-smallest prime number; the next is 11. Seven is a Mersenne prime, since 23 - 1 = 7. Seven is also the fourth Mersenne prime exponent, a Newman-Shanks-Williams prime, and a Woodall prime. 7 is also the second lucky prime.
999,999 divided by 7 is exactly 142,857, so fractions with 7 in the denominator have six-digit repeating sequences in their decimal expansions. 1/7 = 0.142857142....
A seven-sided polygon is a heptagon. The regular n-gons for n ≤ 6 can be constructed by ruler and compass alone, but the regular heptagon cannot. Figurate numbers representing heptagons (including seven) are called heptagonal numbers. Seven is also a centered hexagonal number.
There are seven frieze groups, the groups consisting of symmetries of the plane whose group of translations is isomorphic to the group of integers.
In numeral systems
- The number of objects in the solar system visible to the naked eye (Moon, Mars, Mercury, Jupiter, Venus, Saturn, Sun). A fact that is believed to have led to the concept of seven days of the week first adopted by the Babylonians.
- Messier object M7, a magnitude 3.5 open cluster in the constellation Scorpius.
- The New General Catalogue object NGC7, a 14th magnitude spiral galaxy in the constellation Sculptor
- The Saros number of the solar eclipse series which began on -2590 April 8 and ended on -1310 May 16. The duration of Saros series 7 was 1280.1 years, and it contained 72 solar eclipses.
- The Saros number of the lunar eclipse series which began on -2595 July 15 and ended on -1008 February 22. The duration of Saros series 7 was 1586.6 years, and it contained 89 lunar eclipses.
In other fields
Seven is the smallest positive integer requiring two syllables in English.
There are seven musicians in a septet and seven babies born in a set of septuplets. The most famous set of septuplets are the McCaughey Septuplets, who were born in 1997.
The British fifty-pence and twenty-pence coins are heptagons, with the sides curved to give them a constant diameter.
Many cities are claimed to be constructed amidst seven hills; see Seven hills of Rome and List of cities claiming to be built on seven hills.
Seven is also:
- the largest number of sticks (or other cylindrical objects) that can be tied into a bundle such that the shape of the bundle remains fixed. This may have led to the number being viewed with mystical significance by ancient man.
- a highly symbolic number in the Hebrew Bible, being, for example, the day on which God rested in Genesis
- the number of ayat in surat al-Fatiha
- the number of heavens in Islamic tradition
- the number of spheres in the Ptolemaic system
- September was the seventh month in the ancient Roman calendar. After the reform that led to the current order, the seventh month is July.
- in Astrology, Libra is the 7th astrological sign of the Zodiac.
- Boomer Esiason's jersey number.
- the traditional number of Wonders of the Ancient World.
- viewed as a lucky number in many Western cultures, and in Japanese culture.
- the number of days in a week. Whether Saturday or Sunday is the seventh day varies across cultures.
- Septidi was the seventh day of the decade in the French Revolutionary Calendar
- referred to in bingo as 'Lucky Seven'.
- the number of the Deadly Sins: lust, avarice, envy, pride, sloth, gluttony and anger.
- considered the usual number for a witches' coven.
- the number of palms in an Egyptian Sacred Cubit.
- the number of the ages of man into which William Shakespeare divided a lifetime.
- the figurative number of seas.
- the number of colors of the rainbow.
- the number of basic principles of the bushido.
- the number of heads of the beast of Revelations, and of some other monsters, like the hydra.
- the number of points on a sheriff's star.
- the code for international direct dial phone calls to Russia and Kazakhstan.
- the name of an elite agency of photojournalists founded by seven leading photographers, including James Nachtwey and others.
- the title of:
- the name of one of the monsters in the cartoon television show Seven Little Monsters.
- the name George Costanza desired to give his first-born (with Susan Ross ) due to a promise he had made to the widow of Micky Mantle , whose number was 7.
- the number of digits in an American telephone number, excluding the area code.
- the number of "Zoomers" on the PBS children's show Zoom.
- the number of sacraments in the Roman Catholic faith.
- the number of individual segments in one digit on a digital clock or calculator screen.
- part of the names of:
- The traditional count of Basque provinces as expressed in the slogan Zazpiak Bat.
- In rugby union, the openside flanker.
- In baseball, seven represents the left fielder's position.
- In all Microsoft Office programs, the F7 key calls up the spelling and grammar checker
- Historical years: 7 A.D., 7 B.C., or 1907
- In music, the roman numeral vii is the subtonic (bVII) or leading tone (VII) scale degree, chord, or diatonic function, when distinguished V = major and v = minor.
- In most Sonic the Hedgehog video games, the number of Chaos Emeralds.
- In Galician folklore, the seventh son will be a werewolf. In other folklores, after six daughters, the seventh child is to be a son and a werewolf. In other European folklores, the seventh son of a seventh son will be a vampire.
- Sevens is a card game.
- 007, codename for James Bond, a fictional secret spy agent.