And make you little girls talk outta your head!"
The number seven is often considered magical, and in folklore, the seventh son of a seventh son is often considered to have inherent magical powers like second sight. In fiction, and sometimes song, this is often shortened to just the seventh son, though seventh sons were common enough (up until the 20th Century) that this makes little sense.
Characters often claim to be a seventh son of a seventh son, to help convince others that they have magical powers, even in stories where magic is not real.
Traditionally, this trope was Always Male, possibly because "seventh daughter of a seventh daughter" doesn't quite roll off the tongue. (Though it's more likely for less charitable reasons.) Modern works, however, have an increasing tendency to feature the distaff version—especially in Feminist Fantasy, where the author may explicitly add a "take that!" comment about the traditional version.
- Johnny Thunder, an early member of the Justice Society of America, got his powers from being the seventh son of a seventh son, born on the seventh hour of the seventh day of the seventh month of 1917.
- The villain Man-Witch from Common Grounds is a deliberate case; when his mother found out her husband was a seventh son she decided to have seven sons to see if it worked. It did. Incidentally, it really is seven sons; they had somewhere around fifteen children because girls kept messing up the math.
- In Fables, the prophecy concerning Snow and Bigby's seven children claims that the seventh child will live to ages old and is blessed by heaven. This child is almost certainly the zephyr Ghost, the youngest of the siblings.
- In Weres Harry Tom Riddle's shade was extremely upset that Ginny had given his diary to Luna.
"...The Weasley brat is the seventh child and first daughter born in seven generations. Sacrificing her soul while absorbing her magic would have been enough to give me life of my own..."
- In Son of the Desert Trisha is the seventh daughter of a seventh daughter, a batsheva which means she has a sacred place in Ishvalan society. Everyone is allowed to call her by name, marrying her will allow someone to start a new family name and she's free to speak her mind around anyone. She marries the immortal Hohenheim.
- In Ernest Scared Stupid, Old Lady Hackmore says "you're the seventh son of the seventh son" when listing all the auspices that make Ernest The Chosen One for fulfilling his ancestor's legacy and defeating Trantor the Troll once and for all. (It's not clear if she's being metaphorical or not, though, especially since she refers to him as the direct descendant of Phineas Worrell.)
- In Hope and Crosby's Road to Bali, they sing "Hoot Mon", which has the lyric "The seventh son of the seventh son of a son-of-a-gun from Perrrrrrrrrth".
- In The Seeker, Will is the seventh son of a seventh son, and destined to save the world from darkness.
- In Seventh Son, the trope is unsurprisingly played straight as Tom is the seventh son of a seventh son. At the same time, he's the offspring of a human father and a witch mother, making him a unique combination with astounding potential.
- In The Balanced Sword, Tobimar Silverun is "Seventh of Seven", the seventh child of a seventh child; he completes a quest that his family has been unsuccessfully attempting for centuries, and in the process learns to unlock the power he's inherited from a divine ancestor.
- In Susan Cooper's novel The Dark Is Rising, Will is the seventh son of a seventh son, and was born with a caul. He has a major role to play in the battle between the Light and the Dark.
- In the Daughter of the Forest, the first book in the The Sevenwaters Trilogy:
- Sorcha is the seventh daughter of Lord Colum (himself a seventh son). Her six brothers are bound in a curse to be forever swans by her stepmother that only the seventh child can lift.
- Oonagh and Colum's son Ciarán is the seventh son of a seventh son. Even Sorcha herself comments on this when she hears about the birth.
- In the Discworld series, the magic number is eight, not seven, so the eighth son of an eighth son is a wizard.
- In Equal Rites, Eskarina would have been an eighth son of an eighth son if she hadn't been born a girl, which causes quite a lot of trouble when a wizard mistakenly bequeaths his powerful wizard's staff to her under the assumption that she would be an eighth son. Women are supposed to be witches, not wizards, so most of the story is spent trying to figure out where exactly she fits in the world.
- In Sourcery, we learn that the eighth son of an eighth son of an eighth son becomes a super-powerful "sourcerer", a wizard that is actually a source of magic, and almost too powerful for the world to bear. (This is one of the reasons wizards are discouraged from having sex.)
- Earthsea: It's only mentioned once, but Ged, who grows up to be a very powerful mage, has six older brothers.
- In Alethea Kontis' Enchanted, Sunday is the seventh daughter of a seventh daughter. This gives her a special fate.
- In The Forgotten Beasts of Eld, when Sybel is puzzled that Coren, an ordinary human, knows so much about the magical beasts under her care, he simply explains that he's the seventh son of a seventh son.
- A variant in the Piers Anthony novel Fractal Mode - Nona is the ninth born of the ninth generation, granting her the ability to have truer and stronger magic then other girls, and the potential to flip the magical patriarchy to a matriarchy. Which will flip again when there is born a male ninth of the ninth.
- In Patricia C. Wrede's Frontier Magic, 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. The Always Male part of the trope is averted by the series, as double-seventh daughters are also magically gifted. It just takes a while for anyone to realize that the seventh daughter of a seventh son counts as a double-seven - all previously recorded double-sevens are seventh sons of seventh sons or seventh daughters of seventh daughters.
- 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. On the other hand, what it causes him doesn't seem very lucky at all...
- In Groosham Grange, wizards are all seventh sons of a seventh son and witches are all seventh daughters of a seventh daughter.
- It doesn't come up as an important plot point in Harry Potter, but Word of God notes that Ginny being the seventh child in her family (albeit the first daughter) was supposed to symbolize her as someone special. She is also the first girl born into the Weasley family in several generations.
- A seventh son of a seventh son can allegedly see the torcs of Droods in the Secret Histories series. In Live and Let Drood, a seventh daughter of a seventh daughter proves she can, too.
- In The Secret of Platform 13, Odge the hag is the seventh daughter of a seventh daughter. Her mother was eagerly expecting what would surely be her most terrifying child yet, and was disappointed when she came out looking remarkably human, except for heterochromia, one blue tooth, and a small bump that hopefully will grow into an extra toe one day.
- In The Secrets of the Immortal Nicholas Flamel, Perenelle Flamel is a female example. Being the seventh daughter of a seventh daughter allows one to speak to ghosts.
- In Septimus Heap, Septimus is a powerful wizard, and the seventh son of a seventh son.
- In Joss Stirling's Soulfinder Series, among savants, seventh children are always more powerful than average. The most prominent seventh children are Zed Benedict, who actually is a seventh son, and his mother Karla. They both have the ability to read minds and see the future.
- The third book, meanwhile, introduces Crystal Brook, who as the seventh child of her family is supposed to be powerful, but only has the ability to find things. Not only that, but she can't do telepathy (a universal savant ability), and it actually makes her nauseous. It turns out that Crystal is a soulseeker, which is an incredibly rare power. Soulseekers can, essentially, see the Red String of Fate that connects every savant with their soulfinder (soulmate), and thus bring them together. No one realized this before because a)soulseekers are incredibly rare, so none of her family had met another one, and b)her inability to do normal telepathy, which seems to be unique to her.
- Taken Up to Eleven in Orson Scott Card's The Tales of Alvin Maker. Alvin is not only a seventh son, he is the seventh son of a seventh son, his parents' thirteenth child, and to top it all off was born with a caul, and is a powerful "maker" — a rare and powerful form of magic. The first novel in the series is even named Seventh Son.
- Alvin's younger brother, interestingly, is also (sort of) the seventh son of a seventh son, because their oldest brother died just after Alvin was born. Calvin doesn't have quite Alvin's level of power, but he's more powerful than average by a good bit.
- And their father, Alvin Sr., is also above average.
- In Mercedes Lackey's Tales of the Five Hundred Kingdoms series, Sasha has some small magics, in part because he's a seventh son. Also, in One Good Knight, Gina mentions that three of her siblings have magical abilities.
"That kind of startled my parents, they had no idea that there was magic in their blood,but the sibs in question are the seventh born, the ninth born, and the thirteenth born, so that probably explains it."
- In The Wardstone Chronicles, being the seventh son of a seventh son is literally a job requirement to be a spook.
- It's mentioned in Wicked that Elphaba's preacher father is the seventh son of a seventh son and that he comes from a line of six ministers. He fully expected his firstborn to be a son, but he instead got a daughter. Elphaba's sex, along with her green skin, are just the start of Elphaba's life of screwing with her father's expectations.
- Worldweavers: Thea, as the seventh child of two seventh children, is supposed to have powerful magic. She's so bad at magic that her parents wind up sending her to a boarding school that specializes in people who can't do magic (in the world of the series, almost everyone can do magic). She's later revealed to have command over two of the four classical elements, fire and air. This is extremely rare. And her inability to cast normal spells? It's because she can only cast through a computer, which is also rare.
- In Charmed, a seventh son of a seventh son is revealed to be The Chosen One to defeat a powerfull witch, by using her own wand against her.
- In the Soap Opera Days of Our Lives, a shadowy crime lord called "The Phoenix" claims to be the seventh son of a seventh son and to have magical powers.
- Doctor Who: "Terror of the Zygons" has a landlord who claims to have the power of second sight because he's the seventh son of a seventh son.
- In Highlander, the fortune teller mentions Duncan being this. The mystical part is probably in connection with his immortality.
- In The Twilight Zone episode "Still Valley", a Confederate soldier met an old man who had magical powers because he was the seventh son of a seventh son. He also had a Deal with the Devil thing going on.
- Willie Dixon's classic and much covered blues song, "Seventh Son" claims various powers for the singer.
- Led Zeppelin's song "Poor Tom" recounts a middle-aged laborer named Tom, who was the seventh son of a seventh son, and could see reality as it really is. This leads him to kill his promiscuous wife.
- Iron Maiden's album Seventh Son of a Seventh Son, and the title track, are all about this trope; the album was inspired by Orson Scott Card's novel, Seventh Son (see Literature).
- From The White Stripes' "Ball and Biscuit":
I might be your third man, girl
But it's a fact I'm the seventh son.
- From Tears for Fears' "Raoul and the Kings of Spain":
When the seventh son of the seventh son
Comes along and breaks the chains...
- 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.
- In the tabletop role-playing game Changeling: The Lost, being the seventh son of a seventh son is one of the conditions that can help someone penetrate a Changeling's glamour.
- In Mage: The Awakening, "Magical Traditions" introduces Southern Conjure as a "flavor" which offers a special merit called "Seventh Son of a Seventh Son" that basically turns any mage into a walking, talking Fate Arcana magnet.
- The New World of Darkness book Skinchangers gives us the Lobison, an Argentinian werewolf story wherein the seventh child of a family, if a boy, will be cursed to change into a bloodthirsty wolf-creature when stressed. (In real life, the tale of the lobison led to so many children being abandoned or killed that the government had to step in.)