Treehouse of Horror is The Simpsons' Halloween special: at least one episode a season, every season since 1990, usually told as three short horror stories that sit outside the show's continuity. Characters die, Springfield is destroyed, aliens invade, and next week everything is back to normal. El Barto is Bart's graffiti tag, and Krusty the Clown is the children's entertainer Bart worships.
Below: how the Halloween episodes work, the segments people still quote, who Krusty and El Barto are, and Homer's lifelong romance with the donut. Every design on The Simpsons shelf comes from somewhere in it.
What Treehouse of Horror is
The name comes from the first one, in the second season: Bart and Lisa sit in their treehouse on Halloween night and swap scary stories, and each story plays as its own short. The treehouse soon dropped out of the episodes, but the name stayed, with a Roman numeral for each year.
Because nothing counts, the writers can do anything. The family dies, the world ends, Homer becomes a monster or meets one. The opening credits swap in spooky versions of the names, and many segments parody a famous horror story, film or episode of a classic anthology show.
Kang and Kodos of Rigel VII
Kang and Kodos are two green, one-eyed, tentacled aliens from the planet Rigel VII who travel in a flying saucer and drool constantly. They first appear in the very first Treehouse of Horror, in "Hungry Are the Damned", when they abduct the family and feed them lavishly. Lisa is sure they are being fattened to be eaten and finds a book that seems to prove it. The aliens insist they only wanted to be good hosts, and they leave hurt.
After that, the two turn up in almost every Halloween episode, sometimes in a full story, sometimes in a single shot. In one later outing, "Citizen Kang", they take the places of the two candidates in a presidential election. Hail Rigel 7 has them with Homer.
Zombie Springfield, the Devil and a donut
In "Dial Z for Zombies" (the third Halloween episode), Bart finds a book of magic in the library and tries to raise the family's dead cat. He raises Springfield's dead instead, and the zombies, Ned Flanders among them, go after the living until Bart finds the spell to put them back. Both zombie designs on the shelf, Treehouse Night and Springfield Undead, put Homer himself among the undead.
In "The Devil and Homer Simpson" (the fourth), Homer says he would sell his soul for a donut. The Devil appears, and he is Ned Flanders. Homer makes the deal, eats almost all of the donut and tries to keep the last bite to save himself, but eats it anyway. At the trial for his soul, Marge produces proof that Homer had already promised it to her, so the Devil loses. On his way out, the Devil turns Homer's head into a giant donut.
On the shelf
Krusty the Clown, and Itchy and Scratchy
Krusty the Clown hosts Springfield's children's television show and puts his face on everything from cereal to the Krusty Burger chain. On camera he is all "Hey hey!"; off camera he is tired, cynical and often in debt. Hey Hey is the on-camera Krusty.
Born Herschel Krustofsky, he is the son of a rabbi who disapproved of his clowning, and the two later make up. In the first season, his sidekick, Sideshow Bob, frames him for the armed robbery of the Kwik-E-Mart, and Bart, his biggest fan, proves the frame. That starts Bob's long hatred of Bart.
Krusty's show also runs Itchy and Scratchy, a cartoon in which Itchy the mouse kills Scratchy the cat in extravagant, bloody ways, to Bart and Lisa's delight. Fight, Fight, Fight is the pair at it. Krusty also gets his own Halloween short: in "Clown Without Pity", Homer buys Bart a talking Krusty doll that has been left switched to "evil", and it tries to kill Homer.
On the shelf
Mmm donut, and the town of Springfield
Homer's drooling "Mmm..." is a catchphrase that works with any food, and the donut is the one he returns to most, often pink-iced with sprinkles. Mmm Donut is that moment.
Homer works as a safety inspector at the Springfield Nuclear Power Plant, owned by Mr Burns. The family lives at 742 Evergreen Terrace, next to the Flanders, and Homer drinks at Moe's Tavern.
The show never says which state Springfield is in. Maps, borders and neighbours shift from episode to episode, and every hint is a joke at the expense of anyone trying to pin it down.
What El Barto means
El Barto is Bart's graffiti tag: his name with a Spanish-style "El" ("the") in front, the way a tag is meant to look. The tag first appears in "Homer's Odyssey", in the first season, and turns up on walls and buildings all over Springfield.
The town treats El Barto as an outlaw whose identity is a mystery. Only a few characters, Principal Skinner among them, learn who is behind it. El Barto puts Bart and his tag together.
On the back of a tee
All seven designs are on The Simpsons shelf, drawn from canon reference art. Check the size guide before you pick a size.
Why it works on the back: yellow skin, thick outlines and flat colour were made to read on a small screen, so they read just as well across a street.
Read next: The Citadel, C-137 and Evil Morty: Rick and Morty explained and Mystery Inc. and the men in masks: Scooby-Doo explained.
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