Rule of Seven

Right after three, seven is the most commonly used "ominous" number in fiction. This can be traced back to the ancient times, just think of the Seven Wonders, the Seven Days of Creation, or the Seven Hills of Rome. It is also common in many fairy tales.

In the natural world, one of the origins of this significance come from the seven celestial bodies that could be seen with the naked eye, (Sun, Moon, and the planets up to Saturn), that were worshiped as deities, and the days named after them were the days of the week.

Later, natural phenomena were intentionally shoehorned into this "significant" number, for example the colors of the rainbow or the number of continents can be seven, depending on how you count them.

Since The Bible, especially the Book of Revelation, uses it repeatedly, seven became even more famous as a divine number, or as the number of perfection.

Lucky Seven is a subtrope about examples where it is considered a lucky number.

Anime and Manga

 * Seven Dragon Balls.
 * Seven Rainbow Crystals (nah) in the Sailor Moon anime.
 * Seven Celestial Warriors.
 * Seven Warlords of the Sea. There were also seven Straw Hat Pirates for a considerable length of time (then they gained more crew members).
 * Seven Swordsmen of the Mist. Also, the ninja academy graduates each year are assigned to numbered squads. Guess which squad the main character ended up in.
 * Nana uses this rather prominently: both of the main characters are named Nana (a Japanese reading for the number 7), and they share an apartment numbered 707. Subverted in that neither of the two main characters' names use the kanji for 7.
 * The Sexy Seven from Claymore?
 * In Fairy Tail, all the dragons (except, apparently, the jerkass black dragon) disappeared seven years ago. On July Seventh of the year 777.
 * Shuya Nanahara from Battle Royale, with the "Nana" in his last name meaning seven. Used to be nicknamed "Wild 7" when he played baseball, which is also the brand of cigarettes Shogo Kawada smokes.
 * Seven types of Dying Will Flames. Because of this, all important groups of people also come in sets of seven.
 * Yu-Gi-Oh GX has Johan's Crystal Beast monster spirits; when all 7 are gathered he can summon his ace monster Rainbow Dragon. (Its evil twin Rainbow Dark Dragon works in a similar fashion).
 * Inverted in Yu-Gi-Oh 5 Ds, where Season 1 antagonists the Dark Signers and their signature monsters, the Earthbound Gods, number seven in all.
 * The seven Millennium Items in the original Yu-Gi-Oh.
 * Pokémon: Jirachi Wishmaker: Wishing Star of the Seven Nights.
 * Saint Seiya has seven Odin Saints (each guarding one of the seven Sapphire Stones), as well as seven Marine Saints.
 * Yu Yu Hakusho has the villainous seven espers, the seventh of which has seven personalities.
 * Nausicaa of the Valley of the Wind takes place 1000 years after the world was nearly destroyed by the God-Soldiers in the "Seven Days of Fire".

Comic Books

 * DC's Seven Soldiers of Victory, both in the Golden Age and in the anthology series by Grant Morrison.
 * In Seven Soldiers #0, a team is assembled with only six members because one didn't show up. Dan the Dyno-mite remarks that the significance of the number 7 can't be overstated, especially since 6 is extremely unlucky in comparison.
 * Grant Morrison also did a Justice League arc after he expanded the team centered on the number seven, ranging from seven villains attacking Washington DC to Martian Manhunter calling in the Atom because he was the seventh "new member" since the League's inception.
 * "The Seven" are the main superhero team in The Boys
 * Seven Evil Exes. Gideon takes the "seven" motif Up to Eleven. He then takes it up to twelve in The Movie, even stating after he swallows his gum that "It's gonna be in my digestive tract for SEVEN YEARS!"
 * The Mexican superhero Kaliman is The Seventh Descendant of the Seventh Dinasty of the goddess Kali, and must renew his vow to fight injustice every seven years.
 * The seven Endless from The Sandman; Destiny, Death, Dream, Destruction, Desire, Despair and Delirium.
 * The seven New Guardians from Green Lantern; Atrocitus, Larfleeze, Sinestro, Hal Jordan, Saint Walker, Indigo-1, and Carol Ferris.

Fairy Tales

 * "Snow White and The Seven Dwarfs"
 * As a direct consequence of above, seven Leafe Knights in Prétear

Film

 * The Magnificent Seven.
 * And The Seven Samurai, upon which the previous was based.
 * 007.
 * Subverted in Unstoppable: Train #777 (888 in Real Life) is loaded with hazardous chemicals and speeding towards a populated area.
 * Seven's the number. Like a mouse on a wheel.
 * In Hot Fuzz, Nicholas Angel's badge number is 777.

Literature

 * Harry Potter:
 * Invoked by Voldemort, who planned on splitting his own soul into seven pieces — six Soul Jars plus the piece still in his body.
 * Seven books for seven years at Hogwarts.
 * Seven Weasley children.
 * This would have culminated with book 7 being released on 7/7/07, but the bomb attacks on London the year before put a damper on that, so the publisher released it on the 21st (7+7+7) instead.
 * Practically every number you encounter in the Septimus Heap series is either somehow related to the number seven (often the result of adding the numbers together), or an homage to something or other. There's even a list at the end of the first book.
 * Subverted in Discworld, where the most magical number is 8.
 * And the most unfortunate number. 7a dude, 7a.
 * In Dan Abnett's Gaunt's Ghosts novel The Guns of Tanith, Soric reveals that his great-grandmother was a witch, and that his father said it would come to him, as the seventh son of a seventh son.
 * Gilbert L. Morris' books "The Seven Sleepers" series is about seven teenagers who have adventures in a post-nuclear attacked world.
 * Lampshaded in Lord Darcy stories, where the characters are making fun of a sect for thinking 7 is a magical number - in that world's magic, only five - and, to a lesser extent, six - are.
 * In Patricia C. Wrede's Thirteenth Child, Lan is the seventh son of a seventh son, and so a natural magician. Others point out to Eff that she's a seventh daughter, even if she is a thirteenth child.
 * In Orson Scott Card's Alvin Maker saga, the titular character is a seventh son of a seventh son, and this is why he's a Maker.
 * The Lord of the Rings: The Seven Rings of the seven Dwarf Lords; also a Shout-Out to "Snow White and The Seven Dwarfs".
 * And the seven sons of Fëanor.
 * The flag of the kings of Gondor depicts seven stars with the White Tree, and there are seven palantíri.
 * Night Watch has seven levels of Twilight.
 * Keys to the Kingdom by Garth Nix has seven Morrow Days who are Trustees of seven Parts of The Will which each represent one of the Seven Heavenly Virtues and wield seven magic Keys, who each hold jurisdiction over one of the seven days of the week and are in charge of one of the seven parts of The House and due to breaking their contracts, each suffer from one of the Seven Deadly Sins.
 * A Song of Ice and Fire: The Faith of the Seven is all about this : heptagon-shaped temples (called septs, "sept" meaning seven in French), seven gods (who are also the seven faces of one god), seven hells. Other examples include the Seven Kingdoms (a single kingdom that was previously made of seven) with seven major houses ruling what used to be independant kingdoms (with the exception of the Riverlands, but this would become a complete digression), the seven Iron Islands, and there are surely many more to list.
 * George RR Martin plans on making the series seven books. Whether he'll actually be able to do that is a whole other story.
 * Will Stanton in The Dark Is Rising is the seventh son of a seventh son. Although he doesn't actually know that at first, as his parents never told him his oldest brother died in infancy long before he was born.
 * In Edgar Rice Burroughs's The Monster Men, von Horn has not set foot on a place under American jurisdiction in seven years.

Live Action TV

 * Extreme Makeover: Home Edition built seven homes for seven familes in seven days in their finale.
 * The significant Seven
 * Subverted:
 * Seven mysterious Plot Coupons in Defying Gravity, each located on a different astronomical object.
 * Survivor season seven winner Sandra Diaz-Twine became the first (and, so far, only) two-time winner by winning season 20, seven years after her first win.
 * Doctor Who's A Good Man Goes to War was the 7th episode in its series, and the 777th in the show's nearly fifty-year run. Three days after it aired, on June 7, the BBC officially announced Series 7.
 * This was later followed by one of the Executive Producers announcing her departure from the show, at the end of the season, on 7/7.

Music

 * "Seven days in a week made to choose from /But only one is right for me..."
 * "The seven is the number of the young light /It forms when darkness is increased by one"
 * French rap group IAM wrote an entire song about the symbolic virtues of the number seven.
 * Rory Gallagher's Seventh Son of a Seventh Son tells the story of a man born with faith-healing powers, only to lose them in exchange for fame and fortune.
 * Iron Maiden's Seventh Son of a Seventh Son. The opening track even goes: "Seven deadly sins,Seven ways to win, Seven holy paths to hell, and your trip begins..."
 * With Märchen being Sound Horizon's seventh "story album", Revo amused himself by making use of the number seven constantly (over a hundred times, in his own words) throughout it: The album's story revolves around Märchen granting revenge to seven dead women from seven fairy tales, each associated with one of the Seven Deadly Sins. One of them is "Snow White and The Seven Dwarfs". The lyrics booklet is seventy-seven pages long, and the number seven always appears in red.

Video Games

 * Bowser has seven kids... before Bowser Jr., anyhow.
 * Seven Star Pieces in Super Mario RPG: Legend of the Seven Stars.
 * Seven Star Spirits in Paper Mario.
 * Seven Crystal Stars in Paper Mario: The Thousand-Year Door.
 * Super Paper Mario has eight Pure Hearts... but you start the game with one of them, so you've still got to get seven.
 * Seven Chaos Emeralds in Sonic the Hedgehog.
 * There were six Chaos Emeralds in the first Sonic game. Anyone want to guess what inspired the change?
 * Even when it's not Chaos Emeralds, there are seven MacGuffin-style items to collect. Seven Rings in Hand, anyone?
 * Seven Princesses of Purest Heart from Kingdom Hearts.
 * Seven needles in Mother 3.
 * Seven personalities in Killer 7, along with a revolver that has seven chambers.
 * The game is divided into seven chapters.
 * In Resident Evil 4, a gun named Killer7 costs 77700 ptas, and has initially a seven-bullet capacity (which can also be upgraded by increments of seven).
 * Don't get a Halo fan started on the number seven. There have been whole groves of Epileptic Trees over how almost every arbitrary number in the series is somehow related to 7. One of the more obvious ones is 117, Master Chief's operating number, and that there are seven of the eponymous Halo rings.
 * This has been a Running Gag at Bungie since the Marathon trilogy. Consider.
 * All over the place with Punch Out Wii's version of Aran Ryan. He always gets up from a knockdown on the seventh count, you can hit him seven times when he's stunned, and sevens pop up in his challenges in Exhibition Mode. He's Irish, so it's a "lucky seven"/"luck of the Irish" gag.
 * Final Fantasy:
 * Final Fantasy VII (figures) has something interesting happen if a character's HP hits 7777. The Lucky Seven concept continues in the prequel Crisis Core, where getting three Sevens on the DMV (the battle system's slot-machine gimmick) will level up your character.
 * Final Fantasy IX contains the attack "Lucky 7s", a Shout-Out to the above.
 * The seventh Touhou game is the only one to feature the "Supernatural Border" system, which can and will save your life on occasion. It is also the first game in the series to have visible hitboxes.
 * Also, most of the games have a total of 7 stages - 6 normal and the Extra. The 2 exceptions are So EW (with only 6), PCB (Phantasm = 8), though you can technically consider IN as having 9! stages. (there's 2 variants on stages 4 and 6. And no, the game DOES NOT have a certain ice fairy in it.)
 * Stage 7 of Battle Garegga is home to a very popular scoring trick involving using bombs on the penultimate boss's grenades.
 * DJMAX Technika has a song called "Area 7". In an inversion of this trope, it's one of the less-liked songs in the game.
 * One of the best Tetris the Grand Master players in Japan goes by the initials 777.
 * There are seven tetrominoes: I, T, O, S, Z, L, J.
 * In Gradius Gaiden, stage 8 of the first loop has six bosses. On the second loop, it gains a seventh boss, Heaven's Gate.
 * In Pokémon, the best you can get at the slots in the Game Corner is triple sevens.
 * In Pokémon FireRed and LeafGreen, the Sevii Islands are are seven normally accessible islands (though certain event lets you access two more) and they were all created in 7 days, according to a lady on Quest Island, the seventh island. Quest Island itself also houses the seven botany-themed Tanoby Chambers, which are chock full of Unown and shrouded in mystery. Additionally, there are seven beta "Sevii Islands" that are not accessible during normal gameplay.
 * Pokémon Emerald Version introduces Gym Leader-like individuals known as the seven Frontier Brains, each of whom heads a unique battling facility. To attain success in said facilities, one must repeatedly win by defeating sets of seven Trainers in a row. There are a few exceptions, though.
 * As of Generation IV, Eevee has seven evolutions.
 * In Generation V, the villainous Team Plasma includes a group of high-ranking members known as the Seven Sages.
 * The Nintendo DS is also the only Nintendo handheld which has had seven main series Pokémon games/versions developed for it.
 * Pokémon Mystery Dungeon: Explorers of Time, Darkness, and Sky all feature the Seven Treasures, items which are obtained by defeating seven bosses that guard seven secret dungeons.
 * The Legend of Zelda series has seven sages in A Link to the Past and Ocarina of Time, and there were Seven Sages in Twilight Princess before  killed one of them.
 * The following games have exactly seven dungeons/stages each: The Adventure Of Link, The Wind Waker, Spirit Tracks, and Skyward Sword.
 * In the Oracle of Seasons, there is a secret that requires the player to find 777 Ore Chunks.
 * No More Heroes references Killer7 with the Lovikov Balls, as there are 49 of them in total (7 x 7), seven are required for a new ability, of which there are seven as well (each of them based on one of Emir's personalities from Killer 7). Also, one of the Dark Side powers are enabled when the jackpot (which appears every time an enemy is killed) hits 777.
 * In the backstory of the Dragon Age games, Chantry lore claims that the Blight appeared on Thedas when seven ancient Tevinter Magisters traveled to the Golden City of the Maker in an attempt to steal his power. Their invasion of the City corrupted it into the Black City, and they returned to Thedas as the first Darkspawn.

Webcomics

 * In American Barbarian, Yoosamon has seven sons.
 * xkcd has a number line strip that says 7 indicates a false statistic
 * In Erstwhile, Maid Maleen is locked up for seven years.
 * In No Rest for The Wicked, the soldier has to survive seven years to win his bargain with the devil.

Web Original
"AVGN: Yeah, I bet the game designers were smoking seven different herbs."
 * In The Angry Video Game Nerd's review of Kid Kool (whose full name is Kid Kool and the Quest for the Seven Wonder Herbs), the Nerd says that the game has a story that says that the king has a terminal illness, and that Kid Kool has to find the aptly named Seven Wonder Herbs.


 * That is not all. In addition to these seven herbs, he says that the game has "seven horrible design flaws", which are called "Seven Deadly Sins"; and he even has a name for these flaws as follows:
 * 1. Not Being Able to Go Back -- "One-Way Bullshit"
 * 2. Running Too Fast -- "Two-Gear Diarrhea"
 * 3. Jump Control -- "Jump Fuckness"
 * 4. Skipping Over Water -- "Topside Aquatic Ass"
 * 5. Vertical Scrolling -- "Air Suspension Shit Lift"
 * 6. Invisible Items -- "Inviso-Bitch"
 * 7. Having to Drop Down -- "Free-Falling Fuckballs"

Western Animation

 * Seven original members of the Justice League.
 * On Wacky Races, car #7 is occupied by seven little men.

Religion and Mythology

 * Several religions regard 777 (Rule of Three and Rule of Seven united) as special:
 * According to the American publication, the Orthodox Study Bible, 777 represents the threefold perfection of the Trinity.
 * The Seven Deadly Sins
 * The Seven Heavenly Virtues
 * Seven occurs all over The Bible — seven days of creation; David was the seventh of his father's sons. In Revelation, The Dragon has seven heads, The Beast has seven heads with seven crowns, seven churches in Asia Minor, seven seals, seven last plagues...
 * Many Abrahamic traditons include seven heavens.
 * The Seven Sleepers
 * In mythology, the seventh son of a seventh son is said to be either magical or a vampire. Iron Maiden's Rock Opera Seventh Son of a Seventh Son was about such a child.
 * Any jolly sailor worth his salt will brag about his encounters along the Seven Seas (actually oceans), and brave adventurers in the days of old Arabia will travel across the Seven Deserts.
 * The Seven Lucky Gods of Japanese mythology.
 * The seven Maskim in Mesopotamian Mythology, described in a famous poem beginning, "They are Seven! They are Seven!"
 * The Jade Emperor of Chinese mythology has seven daughters, the youngest (seventh) is the one that has the most stories associated with her.
 * The Qixi festival is held on the 7th day of the 7th month in the lunar calendar, and is supposed to be the day that its two namesake stars (Altair/Cowherder and Vega/Weaver Girl) are closest to each other.

Real Life

 * Seven continents on Earth
 * Seven seas
 * Seven days in a week
 * Which is, in turn, caused by the presence of seven celestial objects regularly visible from Earth — the sun, moon and five planets.
 * Seven colours of the rainbow. This is actually a contrivance of Sir Isaac Newton, a well-known scientist and not-well-known mystic. Observing the spectrum of sunlight produced by his prism, he convinced himself that the area between blue and purple was a separate colour, indigo, because seven colours would fit his numerology better. The joke of this is, he missed a real seventh colour: cyan, between green and blue.
 * As an homage to this trope, the highest jackpots on a slot machine is often assigned to three 7's.
 * 21 (7×3) in blackjack.
 * Seven is the most likely roll of a pair of standard six-sided dice. In craps, you can win by rolling a seven (or eleven) first, and lose by getting seven on any subsequent roll. The fact that seven is the most likely sum for the outcome of rolling two dice (with a probability of 1/6) is likely the origin of the whole "seven is a lucky number" superstition.
 * In chemistry, the pH scale is neutral at 7. And it's not arbitrary at all, since H+ molarity in water is approximately 10^(-7), and pH is a logarithmic scale (though the mole's value, from which molarity comes, is indeed a fixed one).
 * The number of daughters of Atlas in the Pleiades (also called the "Seven Sisters").
 * Almost all mammals have seven cervical vertebrae.
 * A ladybug has seven spots.
 * Seven is the atomic number of nitrogen, which makes up the majority of Earth's atmosphere.
 * Ask people to randomly choose a digit from 0 to 9 and they're more likely to pick 7 than any other.
 * In physics and metrology, there are seven SI base units: Meter (length), kilogram (mass), second (time), kelvin (temperature), mole (amount of substance), ampere (electric current), and candela (luminous intensity).