In fiction, some key pieces of clothing can only be worn by certain people. Shoes are a common choice, but other clothing will do. Thus if it fits someone, it can be used to ID them. Sometimes this can work in Real Life, but it's usually not enough evidence on its own.
Note it only counts when it's wearing it, not using it, or even touching it.
Anime and Manga
- Parodied in the Ranma ½ manga by a story arc titled "Cinderella's Favor" in which a guy has gathered girls to his private island and asks them to step on his head, so he can compare their feet and find the girl who stomped on his head a long time ago (which, of course, is actually Ranma).
- Battle of the Planets: Princess once lost one of her boots during a battle and the enemies try to use it to find her.
- The wish ring in Little Ego, which will only work for the person who manages to slip it on. Naturally this person turns out to be Ego.
- In many Harry Potter fanfics, each pureblood family has a ring that automatically resizes itself to fit whoever becomes head of that family -- and just may kill a pretender.
Films -- Animation
- Disney's Cinderella, the Trope Namer, acknowledges the potential absurdity of the trope: the Grand Duke protests that the slipper could fit any number of girls, to which the King replies that he doesn't care, he's holding the prince to his Exact Words in order to ensure he gets married. The trope is also ultimately subverted, as Cinderella doesn't even manage to try on the slipper before it's smashed... but she is able to produce its match, which serves as even better proof of her identity. That references an often-omitted part of the Charles Perrault fairy tale, in which after Cinderella fits her foot into the slipper, she pulls out the match and puts that on her other foot.
- In Cinderella III: A Twist in Time, Prince Charming himself acknowledges the flimsiness of a shoe size as proof of one's identity, but also points out they don't have any other means of finding Cinderella. His and the king's doubts about this plan become confirmed when Lady Tremaine uses a stolen magic wand to make the glass slipper fit stepsister Anastasia instead.
Films -- Live-Action
- Subverted in Star Trek VI: The Undiscovered Country. The boots of one of the assassins are found, but in the locker of a crewman with webbed feet, and the boots are for humans. They were obviously planted to avoid this trope. Chekov even mentions the aphorism from the "old Russian fairy tale" - "If shoe fits, wear it." He notably fails to look down to make sure that the boots might indeed fit the person he's accusing.
- Used as contributing evidence in Inglourious Basterds.
- Also subverted in Disney's 2015 live-action adaptation of the story -- the slipper is enchanted to refuse any foot but Cinderella's, but the Prince doesn't need it once he sees Cinderella because he recognizes her immediately. (He's also actually the king by that point, and is not bound by an Exact Words promise, so he simply can say "that's her" and no one argues.)
- Book 4 of The Faerie Queene: The winner of the Beauty Contest will be the one who can wear the (supposedly) dead Florimell's golden girdle, as it can only be worn by a Virgin Power, and virginity is beautiful. At least, those were the rules, but when the belt fits none of the girls except Amoret, the judges stubbornly award it to who they think is the most attractive girl anyway (who is actually a clone of the real Florimell), chaste or not, despite the protests of Amoret's Knight in Shining Armor, who is actually a Sweet Polly Oliver. Yes, it's an interesting story.
- In Chronicles of the Kencyrath, the Kenthiar is a collar that only the Highlord can wear—everyone else who touches the inner surface will lose their fingers, or worse.
- In Diana Wynne Jones' The Crown of Dalemark, Mitt is descended from the Adon and destined to become King, and the ring that marks him out as such actually grows and shrinks in order to fit him perfectly on any finger he puts it on. He doesn't realise, because the ring gets swapped with a nonmagical copy.
- Witches Abroad utterly dismantles this trope. Lilith is trying to Invoked Trope the trope for the Cinderella story she's masterminding, and Nanny Ogg and Granny Weatherwax point out that a lot of feet can fit a shoe that size with enough socks... and even if you don't have socks, there must be a lot of people with that shoe size in the city. The only way to use it as a test is if you know who dropped it in the first place....
- Used in Mercedes Lackey's remake of Cinderella, Phoenix and Ashes. Cinderella's wicked stepmother chopped off her pinky finger in the first chapter, so when she leaves her gloves behind at the masquerade ball, there's no question about who they belong to. Doesn't stop her wicked stepsister from trying, though, and coming out to the hero to claim her gloves still loopy from the painkillers.
- Justified in Ella Enchanted: There's fairy blood in Ella's family, so her feet are significantly smaller than most people's.
- In Bound by Donna Jo Napoli, a Chinese Cinderella story, it was justified because the shoe was designed for unbound feet, like main character Xing Xing who grew up without binding her feet.
- Justified in Princess of Glass: the slipper was molded to the exact shape of the girl's foot. And the slipper isn't the real test anyway—the test is whether the prince can pick out the girl he really loves even with his mind clouded by magic.
- One mystery short had a detective telling a suspect that a killer had left a print behind in blood at the scene of the murder, but it was the print of a glove. The suspect pointed out that would not help identify the killer. The detective said the glove had a distinctive characteristic that would identify the killer exactly and asked the suspect to show him his hands. It turns out, the glove was made for someone with six fingers.
- Jim Henson's The Storyteller plays with this trope in the episode "Sapsorrow". First there's a ring handed down through the generations of the royal family; in order to choose a wife, the king must find a woman who fits the ring, and by the same token if the ring turns out to fit a woman, the king must marry her... even if she's his own daughter, demonstrating one of the problems with relying on the fit of a piece of jewelry to choose your wife for you. Later on, the eponymous princess leaves her slipper behind at a ball, Cinderella-style, and the prince makes the standard proclamation that he will marry the girl whose foot fits the slipper - which nearly backfires on him when Sapsorrow's fat, ugly, bad-tempered sister manages to cram her foot into it.
- Mighty Morphin Power Rangers: Several episodes had Bulk and Skull trying to learn the identities of the Power Rangers. In one episode, Bulk took inspiration from "Cinderella" and decided to make imprints of the Rangers' boots. An opportunity presented itself when Lord Zedd's Monster of the Week in that episode had the Rangers battle illusions of past monsters at the beach. At the end of the episode, Bulk and Skull went to Ernie's juice bar with a cement block with imprints of all five Rangers' boots. One of the rangers turned the music on so the musicians would make Bulk drop the block. While he managed to keep holding the block despite the dancers, he was caught unprepared for Skull's congratulatory tap on the back.
- In the Doctor Who episode "The Beast Below", Liz 10 (Elizabeth X, queen of a future Britain) has a royal mask moulded to the exact shape of her face. This leads to the discovery that she has been ruling for centuries, not for ten years as she believed - her memories are erased every ten years to keep her from remembering the Star Whale.
- Parodied in The Goodies episode "Punky Business". Graeme turns Tim into a punk by cutting his leg off. After re-attaching it, he warns Tim that the catch won't last much past midnight. Tim goes to the Trendsetter's Ball, where he loses his leg at midnight. Caroline Kook vows to marry the man whom the leg fits. Cue punks cutting off their own legs.
- Subverted on The Fresh Prince of Bel Air. Will finds a last-minute date to a Halloween party in a kind waitress named Cindy, who just happened to dress as Cinderella. At midnight, she flees and drops her shoe and Will thinks he's been "touched by magic"... but then Cindy comes back to claim her shoe and gives Will her phone number.
- Defied in My Little Pony: Friendship Is Magic. Near the end of "The Best Night Ever", the ponies are running away from the Grand Galloping Gala and Rarity drops a slipper. Pinkie Pie tells her, "Now your prince is sure to find you!" Rarity, who has no interest in meeting Prince Blueblood ever again, screams and crushes the slipper.
- An... interesting variation occurred in Rocko's Modern Life once: When Rocko accidentally mooned a newspaper camera during a movie premiere, a fashion designer saw the picture, and decided the owner of the posterior would make a perfect underwear model. To find the potential model, all of the men in O-Town took turns sitting in an imprint made of Rocko's bottom in the cement that night.
- Lola and Virginia: While wearing a yogurt costume, Lola met a celebrity boy. As she left, she lost her glasses and the boy uses them as a clue to find her. Being interested on the boy, Virginia gets her own yogurt costume but Lola proves the truth by being the one who can see while wearing the glasses.