Interchangeable Antimatter Keys



In Real Life, a lock is a device for making sure an unauthorized person can't get through a specific door. There's generally anywhere from one to four keys made to fit a specific lock, and these keys usually won't fit in any other lock (unless the key happens to be a master key). The lock can be opened and closed over and over again.

Not so in games with this trope! Here, pretty much any key will fit in any door. Sometimes keys will only work in the dungeon you find them in, but still, within each dungeon every key will fit every door.

Naturally, this would make opening all the Locked Doors very easy, except for one strange fact: whenever you use a key to open a door, the key, the lock, and sometimes even the entire door will disappear, right before your eyes. This makes locks almost useless for their real world purpose, which may explain why you only end up finding Locked Doors in dark forgotten dungeons beneath the earth and not on the doors to people's houses.

In a few games, the structure of dungeons may make it so that the interchangeable aspect doesn't ever come up; whenever you get a key, there's only one locked door you can ever reach, and the next key is behind that door. Still, rest assured that if you cheated up some more keys, you'd find the game treats them as interchangeable.

Compare Skeleton Key.

Straight Examples

 * The Legend of Zelda series is a prime user of this trope. In fact, the first two games had a magic key whose only power is that it doesn't disappear when you use it to unlock a door.
 * The first few games also had keys that worked in any dungeon (the equivalent of the key for your front door unlocking Fort Knox). This created problems, where you would become trapped if you used some keys from one dungeon that you didn't fully complete in another dungeon. The first game had purchasable keys as a workaround, but the second did not. Eventually, they simply made keys only usable in the dungeon in which you acquire them.
 * Instead of buying replacement keys, Zelda II the Adventure of Link had a spell which transformed Link into a fairy, enabling him to bypass locked doors by flying through the keyholes.
 * However, in games where there is a key to the dungeon entrance itself, it is usually unique.
 * Also, there are some keys that are required to enter the Boss's chamber (usually simply called the 'boss key'), and in The Legend of Zelda a Link To T He Past, there was the Big Key. The Big Key was reusable multiple times (within the same dungeon) to open any door with a very big keyhole.
 * Gauntlet (1985 video game) had identical keys scattered throughout every level. Players could open doors or locked chests just by touching them if they had at least one key.
 * In the Fire Emblem series, there are 2 kinds of keys: door keys and chest keys. Any door key can open any door, and any chest key can open any chest. If a particular game has lockpicks, they open both.
 * Shadow Dragon has two others: bridge keys (guess) and master keys (open anything, and have five uses each).
 * Keys and locked doors appear in several Mario games, including Super Mario Bros. 2, Super Mario World and Mario vs. Donkey Kong.
 * Wild Arms called these keys "duplicators" and explains that the key could duplicate into the specific key to fit any lock, once. Afterward the transformed key is no good for any other door, so is thrown away.
 * Except, annoyingly, this fact was only explained in the first game, although the entire series used them. If you started with, say, the fourth game, you'd have no idea why something called a duplicator opens magic boxes.
 * In Diablo II, all keys in the game are identical and can be used to open any locked chest in the game. Yes, they disappear afterwards. The only exceptions are the special keys that are sometimes dropped by the Countess, the Summoner and Nihlathak on Hell difficulty. They don't open anything, at least not in the conventional way.
 * A common feature of mods for Diablo II were some manner of gambling-related chests containing magical items. Usually the keys were not just interchangeable but didn't even open the box at all—they had to be transmuted together in the Horadric Cube (although this was generally for technical reasons; Diablo II didn't have any code that allowed for a container item in the traditional sense).
 * While Deus Ex's actual keys are completely normal, its reconfigurable nanolockpicks and electromagnetic multitools fit this trope like a glove, as they both have ludicrous Handwavium-based excuses for making them non-reusable. And not even an excuse for why it can take four to six lockpicks/multitools to unlock a door, while the lock weakens with each try.
 * The third game has the single-use Automated Unlocking Tools, which unlock any system, be it a security station or a keypad lock. There are also Stop! and Nuke! viruses on floppy disks which can be used on any system and cannot be reused.
 * In Landstalker: The Treasures of King Nole does a mix. You carry mostly Interchangeable Antimatter Keys, except when unlocking dungeons. For example: you unlock the heavy stone door of dungeon #2 with an...idol? No, you don't slot it into an empty pedestal, you just "Use" the Idol.
 * You only ever find the key you need in Drakengard, but some instances of this are ridiculous. For example, the first stage requires you to find a key to open the castle's portcullis, despite the fact that you don't lock those things with keys, if at all.
 * Justified in Betrayal at Krondor, in which every item you can pick up has a paragraph-long description (mostly fluff). The description of the standard key says that it's made for a cheap, low-end lock, "probably manufactured by the hundreds", and "likely to snap". Keys (and lockpicks) have a chance to break and disappear when using them, though they don't vanish every time. More important locks have unique keys, which being less cheaply made are fully reusable.
 * Played straight in the Eye of the Beholder series as well as its spiritual successor Lands of Lore (at least the first one). The first Eye of the Beholder game was notable because each level had multiple keys and each could be used interchangeably on most of the level's lock. Since some locked doors eventually connected to the same area it paid to only unlock doors when you need to (thereby saving the trouble of looking for additional keys).
 * The keys in Lands of Lore aren't interchangeable, but they are anti-matter. Also worth noting is one particular dungeon, the White Tower; while most keys in the game are noticeably unique in appearance, this area uses special "Mystic Keys". These keys are all shaped exactly alike and are only differentiated by their color, but since nothing in the Tower is color-coded, figuring out which key goes into which lock is a matter of trial and error.
 * Sphinx and the Cursed Mummy tries to justify its use of Interchangeable Antimatter Keys by making them weights; so long as a "Glyph Key" is in place in a pedestal, the door or action the pedestal activates will keep working. However, this doesn't explain why all pedestals use the same shape of Glyph Key...
 * The first Dragon Quest I game had one-use keys, though later games in the series allowed single keys to open multiple doors (and they were magic, which explains one key fitting every door in the world).
 * Then you run into Fridge Logic in Dragon Quest III, the prequel to the first game. You take your magic key, which can be used infinitely but doesn't open every door in the game (you need the Final Key for that), and show it to the guy who, by Dragon Quest I, is selling magic keys. Presumably, he copies it, but makes them one-time use and capable of opening any door in the world. In other words, his copy attempt failed completely.
 * That, or he's a savvy/shady businessman.
 * Keys in Boktai work this way. Most dungeons have a square key for the square key door, a circle key for the circle key door, etc., but Otenko notes when you first pick one up that the keys were made by an Immortal and will evaporate if you take them to another dungeon.
 * Weights from La-Mulana qualify. They are found everywhere and can only be used once. They can be placed on pedestals throughout the ruins to activate things. Things like doors, ancient mechanisms, and when you least expect it, Death Traps.
 * The Pokémon Mystery Dungeon games are guilty of this as well. In the first games, even if you have a key, it may not be enough to get to the treasure. Sometimes, you'll need a certain move or terrain ability as well.
 * In Final Fantasy IX, the doors in Burmecia are opened with bells, of all things. Moreover, those bells shatter after being used for no adequately explored reason. Since some of these doors lead to fairly important areas, such as the palace and the place people get married, one must imagine that somewhere a government contractor is raking in the gil making disposable bell keys.
 * In Final Fantasy III, you can only open certain locked doors if you have a "Magic Key" (which disappears after use) or a Thief in your party, who can pick the lock. This rarely comes up outside of Goldor's Mansion, though.
 * In Maximo: Ghosts to Glory, there are identical keys for both gates and chests. The sequel, Army of Zin, has separate gate keys and chest keys.
 * The Atelier series often uses a common form of this trope with color-coded chests that require keys, with the rarest chest type needing a key you'll only have access to near the end of the game. Partly averted, since using the key will cause it to vanish, but once you have the key's recipe you can alchemize as many as needed.
 * The ASCII-based DOS game ZZT had a variant of this: Keys and doors came in seven colors, and any key of a given color would unlock any door of that color (and then disappear forever). You also could only carry one key of each color at a time, which led to situations where your way might be blocked by a key (which was the same size as the player, the doors, and everything else) that you could not pick up until you found a door to use the key you already had on.
 * Keys can be indirectly pushed at least (but you can't directly push them).
 * The Subspace Emissary mode from Super Smash Bros. Brawl.
 * The two 3D Klonoa games play this straight ("One key, one door") although it's the variation in which you can only reach one key at any time. The 2D puzzle games avert it, however: keys come in multiple shapes, and each one can open all of the matching doors in that level.
 * Bugdom, although the cheat codes are so ludicrously simple (Shift+Tilde+F1-F9) that it's really not that hard to just cheat your way into a complete set.
 * Neverwinter Nights.
 * The module editor let you make each door work however you wanted, with the keys being interchangeable or not as you desire. You could also set the "relockable", "requires key to unlock" (as opposed to being vulnerable to the Open Lock skill), and "consumes key when used" flags, although not all three at once.
 * Faxanadu not only has this trope in effect, but the keys (with one exception) have to be bought from a village locksmith. There are, however, five different types of keys.
 * Alundra 2 has these.
 * In Little Big Adventure games, keys are intended for doors located in the same area, and when you leave that area without using the key you found there, it'll disappear. However, all keys are identical, as demostrated in areas where there is more than one door to open - getting one key will give you access to any of the doors you choose, but it vanishes after use, and you'll have to get more keys for other doors in the area.
 * Dubloon features such keys.
 * Outcast has a rare case of providing an in-game explanation for disposable keys. There is a popular resistance movement going on, and one of the people who handles crafting of keys mentions making them intentionally brittle to prevent soldiers from getting to their stashes (locks are rare, and practically never used by civilians). The protagonist reacts with appropriate frustration upon hearing this.
 * Team Fortress 2 now has this in the form of Mann Co. Supply Crate Keys. You can get locked crates from the Random Drop System, which in turn can be opened by these one-use keys. The problem? You have to buy the keys. With real money.
 * Lost Odyssey features this in one dungeon with "gate pass" items you have to steal from enemies to deactivate some electric gates. All the passes fit all the gates, and are consumed in the process.
 * Secret Agent has the classic set-up. Colored doors (red, blue and green), colored keys, doors disappear when you walk into them with a key. On some levels it's possible to pick up a key when you've already got one of the same colour, which causes you to waste a key and inevitably makes the level unwinnable.
 * Pocky & Rocky 2 had locked chests and occasionally locked doors that could all be opened with identical keys. Keys could be found in baskets, dropped by enemies, or bought in stores, but if you had Little Ninja as your partner, you could use her magic to pick the locks and bypass the need for keys.
 * Wonder Boy III the Dragons Trap is an Egregious example, as the locks actually look different, but are all opened with the same keys. (The different locks indicate whether a door has to be unlocked just once or every time you open it.)
 * In the Puzzle Platformer Castlequest, a lot of the strategy is finding various colored keys and not wasting them on the wrong doors.
 * That Legacy of the Wizard displays a meter for keys right next to the life, magic and gold meters should show how much this game relies on feeding keys into doors. Fortunately for Meyna, whose area contains long strings of locked doors, there's an item that lets her use magic instead of keys to open doors.
 * In Spyro the Dragon, Spyro only ever needs one key at a time, located a sufficient distance away from the locked box to make things..."fun". Later game A Hero's Tail had things called "Lock-Picks," which you had to buy so that your little dragonfly friend could fly into locks and pick them, but they looked more like keys than like actual lock-picks.
 * In The Binding of Isaac, a key can open any locked door or golden chest. After using, it disappears.
 * Tutankham had keys disappear when used, though the player could only carry one key at a time.
 * Flink has single-use magic keys for opening locked treasure chests. They are selected through a menu like other spells which work quite differently.

Aversions, Subversions, and Other Deviant Uses

 * Ultima Online. Since a key allows accessing a specific player house, it is useless for anything but one specific door. More keys can be made for the same door by copying the pattern from an existing key to a "blank key".
 * Ultima VII: The Black Gate, from which Ultima Online largely derived its style, also featured a unique key for every locked door or chest in the game.
 * Which was a massive pain in the hind-end, because the game had a deliberately difficult inventory system and the character ended up with dozens of keys taking up space in it, each of which had to be hunted down and clicked on in the inventory before opening its target. As a result, the greatest treasure in the sequel's expansion pack was a magic keyring, which simply automagically gathered the keys and gave the character a "use whichever key fits" function.
 * Ultima VII Part Two: Serpent Isle had even more keys. Thankfully its add-in pack Silver Seed introduced a keyring where all keys could go. The keyring could then be used directly on a lock and if would open if the key was on the ring. Ultima Underworld II: Labyrinth of Worlds had already introduced a keyring but in that game the keys still had to be tested individually.
 * That related trope about the locked door always getting opened? Ultima VIII avoided that one. in the later part of the game there's this HUGE set of double doors that just scream "I am big and important! Open me!" There's no key. If you cheated in the game and moved the doors there was nothing behind them. It was apparently supposed to be how you'd get into the expansion. Which never came out...
 * Harvest Moon: A/Another Wonderful Life actually does feature a locked door that will never be opened. It was, of course, a massive source of controversy and easter-egg hunters.
 * Luigi Mansion has door specific keys for the whole mansion and never to be used again.
 * Fable had a variation on this trope with its Silver Keys. Any silver key can be used in any silver chest and are not used up when the chest is opened, but chests require multiple keys. It doesn't matter which keys you use, so long as you have more than the key number of the chest.
 * This would actually be plausible if the chests have multiple identical locks that all have to be opened at the same time, and you are be unable to remove the key while a lock is open. So you'd need one identical key for each lock. But no such multiple locks are visible on the chests.
 * Final Fantasy I featured several items used to grant access to various dungeons that did not disappear when used, even if they only needed to be used once. It also included many doors that were described to the player as "locked with the MYSTIC KEY"—once the player obtained said key, they could access the doors.
 * Sly Cooper, too, featured the collection of keys that did not disappear when used. Keys could only be used in the general world zones in which they were collected, and specific plot items and doors were locked with a number of locks that determine how many keys Sly needed to collect within this zone in order to unlock the next part of the plot.
 * Averted in the Resident Evil series, where certain keys only work in certain doors. The game will notify you when they are useless and can be tossed away, but if you want to, you can keep them indefinitely. They will take up inventory space, though, at least until you put them in one of the item boxes. Of course, this raises the question of how the main character is able to instinctively realize that this particular key will never be needed again...
 * Could be handwaved in some games, where the map tells you which doors need which keys.
 * Played totally straight with the Small Key items in Leon's scenario of Resident Evil 2. The keys are used to open two small desks in the police station, and are gone from your inventory after use.
 * Silent Hill also subverts the "every door opens" aspect by including many, many doors that can never be opened at all. If the lock is jammed, it stays jammed forever.
 * This is such a staple of the Silent Hill series that the newest game in the series, Shattered Memories, uses the ability to open any door as a selling point.
 * The webcomic Kid Radd Lampshaded this with Itty Bitty, a shopkeeper NPC, who had an unlimited amount of such keys, and used them several times to escape imprisonment.
 * In the first Lufia game, the last door just before the Final Boss could not be opened, leaving some to suspect it was a developer room.
 * Lufia II had a similar door in the final area of the game which could not be opened.
 * Lufia and Lufia II avert this trope by having unique keys for every door, and after they're used the doors stay open forever and the keys stay in your inventory.
 * Lunar: The Silver Star has the ever-obnoxious red chests, that can only be unlocked by a specific item much later in the game...and it's not always worth it.
 * In the Steampunk RPG Arcanum, each key is can only be used on specific locks, and continue to exist once they have been used. Due to this, an accomplished thief (or brutal murderer) can collect upwards of 30-40 keys by the end of the game. (They take up a bit of room until you get a key ring, though...)
 * Averted in both The Elder Scrolls: Morrowind and Oblivion. Almost every locked door in the game has a specific key to open it, usually held by an NPC; once you obtain the key, you can use it as much as you like. Due to Good Bad Bugs in the latter game, you'll generally end up with a lot of keys that you can't get rid of by the end; thankfully, there exist mods that place all keys onto a keyring.
 * Played straight with the lockpicks in both games; in Morrowind, they break after a certain number of uses, and in Oblivion, they break if you fail at the minigame. There are also Open Lock spells with varying magnitudes, but some (usually plot-important) doors require keys to open.
 * All Commander Keen games (except Keen Dreams, which has only one type of key) have several colours of keycards or keygems that open a door of the same colour. In case of the gems the gem doesn't actually disappear, it just becomes stuck to the pedestal you place it on.
 * The skeleton keys of Nethack don't disappear on use, which makes them all the more useful - they work on every door and box in the game. The variant Slash'EM occasionally has these keys break in the lock, but eventually you can find up to three unbreakable artifact keys, each one of which unlocks almost everything in your way. The few things they can't unlock...well, that's why there's three of them.
 * The Nintendo Hard, nigh-Unwinnable NES game Castlequest had their keys and doors color-coded. Figuring out how best to spend your keys was part of the challenge.
 * Paper Mario games both play this straight and avert it. A few dungeons (such as the final dungeon in the original and the fifth dungeon in The Thousand-Year-Door) use Interchangeable Antimatter Keys, but in others, you get specific keys which only work in certain places, and some of which may stay in your inventory after use. The "a specific key will only work here" places are generally in the overworld, but many dungeons also have sequence puzzles solved by using specific keys in certain places.
 * And yet you are still asked what key to use out of the keys that fit. Every single time.
 * The second game actually has a lock on the front door to someone's house...because he's lost his key. Finding it for him is one of the game's many sidequests.
 * Beyond Good and Evil has a grand total of four keys in the game. Two of them, the Triangle and Square Keys, are used multiple times across multiple dungeons and side-quests. The third is a duplicate Triangle Key which is needed for one dual switch puzzle; it's a bit pointless after that, but can still be used to open doors. The fourth, the Star Key, does become useless after one sidequest, but never vanishes.
 * In Chip's Challenge there are four key colours. Red, yellow and blue keys vanish when used, but green ones don't.
 * This has the effect of making certain puzzles Unwinnable if the player opens certain locks at the wrong time (for example, opening the doors to the chip door or the goal in Elementary before getting all the chips behind the other doors).
 * Kingdom of Loathing is mainly an aversion, with an extremely weird subversion- there are a few different keys, each used in a specific place- for example, Boris's, Sneaky Pete's and Jarlsberg's Keys open their Shrines in the Big Mountains. However, in the Naughty Sorceress Quest, there's a Perplexing Door that's constantly changing- and can be opened by any key, with a different thing behind it depending which key you use.
 * Although the game text says "any key", not all the keys in the game work there.
 * And alternatively, there are one or two other objects (say, a balloon monKEY) that also fit in the lock.
 * Also lampshaded upon using a certain key, which gives you the message "In typical video game fashion, the key vanishes after having been used once."
 * Skeleton Keys (made from skeleton bones and teeth) unlock any door in the Daily Dungeon, but frequently "break off in the lock" and disappear. A single VIP Key can unlock the VIP lounge of any clan; according to the game, they all have identical locks. For some reason.
 * The Fallout series is probably the best example of realistic key use in games. A key or keycard will only work with specific doors, and you can keep the keys afterwards. You can even find keycards which have no use in game, or find keys well before (or after) you find the door they're used on. Oddly, you can sell some keycards afterwards.
 * One aspect of this trope is averted in the original Star Ocean 1 for SNES; you do see a locked door which never opens. This is simply due to the developers running out of time. In the PSP remake the door does open once you've finished the game.
 * Similarly, Wild Arms 4 has quite a few doors inside towns that, upon examining them, tell you "this door is locked!" One imagines this is merely to provide a bit more environment to the town without having to program extra rooms (despite the fact that Wild Arms 4 has no rooms in villages, just a blurry image with a character sprite on top of it).
 * World of Warcraft has just about every sort of version imaginable. Many dungeons have doors that require a specific key to open, and that key, once obtained, remains with the player forever unless discarded. Certain "keys" don't actually open a door but instead act as Plot Coupons that unlock harder difficulty modes. Still other dungeons have keys that only exist within that dungeon and must be obtained every time the player enters; these are consumed once used or if the player leaves. Many, many quests involve obtaining keys that can only be used for that quest and are consumed as they are used; leftover keys also spontaneously vanish once you complete the quest. There are randomly scattered chests and lockboxes throughout the game; these can be opened by a Rogue with lockpicks, Engineers with blasting charges, or Blacksmiths with skeleton keys. Those same methods may also be used on many—but not all—doors (with no particular explanation for the difference), and only the Rogue's is reusable as it doesn't involve a consumable item. Ironically, the cost of crafting a blasting charge or skeleton key is frequently greater than the value of the items you obtain from using them.
 * In Wizardry VII all keys self-destruct (some has several uses, some one) upon unlocking, but there's mix of "unique" and several "common" keys (latter can even appear in random loot).
 * Quest for Glory IV: Shadows of Darkness sees The Hero, Prince of Shapeir hold on to every key he finds because they only open specific doors and locks.
 * Not all that bad, though, as there are only four keys you can get anyway (your inn room key, the key to Dr. Cranium's lab,, and the key to the adventurer's guild).
 * Mostly averted in Trapped, to an irritatingly realistic extent: Most doors start off locked, with the entirely ordinary keys hidden around the house. When you find a key, you pretty much have to try every door until you find the right one.
 * In Earthbound, after a specific key is found to be too bent to use, the person who gave you it exchanges it for the "Machine that Opens Doors, especially when you have a slightly bad key". This opens the lock in question, as well as another plot-required lock shortly afterwards, and then has no further use ever again. Since it sounds incredibly useful, many first-time players likely keep it in their (very limited) inventory for the whole game.
 * Keys in Geneforge are rare and open specific doors, but "living tools" (lockpicks) vanish when used. However, they're a "deviant use" in the sense of being justified—they're Organic Technology, and they die after being used.
 * Most MUDs have locks and doors that can only be opened by a specific key (unless you can pick the lock).
 * Crash Team Racing averted the anti-matter part, so by the end of the game, you have 4 identical keys, and need to use it on a door with 4 locks on it.
 * Maniac Mansion handles its keys very realistically, although this is helped by the limited number of doors and locks in the game. All the locks are differentiated and require different keys, although there are locks keyed alike (such as the coin boxes in the arcade room), all of the opened locks can be re-locked and all of the keys stay in your inventory (which can be kind of a pain, given that there are three different inventories for three different kids).
 * Also true of the NES game Deja Vu, a FP Puzzle game. There are some 5 or 6 different keys that are used for various doors, etc.--you know, the sort of things you'd expect keys to open. Part of the puzzle is determining which key opens which door. Shadowgate, by the same developers, does likewise.
 * Jagged Alliance games have plenty of keys to be found. The original Jagged Alliance 2 campaign has over 100 different sets of keys to be found, and each unlocks one or more doors, so they never disappear and it can get confusing. Some keys have duplicates, and some are useless (?). However, it is impossible to lock a door once it's been unlocked, and enemies are able to pass through locked (or trapped) doors without any key, effort, or lock-picking equipment. Another interesting aspect is that the character opening the door needs to be carrying the key (no shared inventory), which can be very frustrating during combat.
 * Runescape is an aversion, in that nearly every door has a unique key associated, and most of them are infinitely reusable. Fortunately, there's also a keyring item which lets you store certain important keys so you don't have to waste nearly as much bank space as you might think.
 * But at the same time, it is present and accounted for in a lot of cases in the game as well. Crystal Keys break off in the lock, as do H.A.M. keys, but Shade Keys go one step further by dissolving when used.
 * The hybrid FPS/Driving game Redline didn't even bother with keys at all. In one mission, after coming across a locked door, your Mission Control flat out tells you not to waste your time searching the level for a key and to just blast the door down with a rocket launcher.
 * Averted in Pokémon. Every locked door is opened by a different key, and once you've unlocked a door, the key just sits uselessly in your inventory forever. (Exception: In Pokémon Diamond and Pearl, the key that lets you into the back of Team Galactic's lair explicitly breaks off in the lock, likely to avoid confusion with the key you find there that lets you into the rest of the lair. Justified in that it's an old key and lock.) Also, I'm pretty sure there are locked doors that can never be opened.
 * There are also those several locked doors in the Abandoned Ship in Ruby and Sapphire that each require a specific key to open, but the keys remain stuck in the doors after they are used.
 * Averted in Cave Story, where each key you find is used to unlock a specific door...to be never used again.
 * Averted in Arcanum - every key realistically opens a single door (or chest) and you can store them all on your key ring. The advantage to this is that if you're caught picking locks, you tend to get arrested, but if you have a key (say, to the shop owner's back room) nobody notices you stealing their stuff. No matter how many times you do it. It's a handy way to get items identified for free.
 * Braid subverts this quite cleverly. The keys are indeed interchangeable and single use...but you have time travelling powers. Some puzzles require a specific key be used on a specific door even though they appear to be interchangeable, while others need you to open multiple doors with a single antimatter key.
 * Wolfenstein 3D plays around with this. You can open any locked door in a map as often as you want after you find the necessary key. However, once you exit the level, you lose all your keys and usually have to find them again for the next map. This is also the case in Doom and several games on the Build engine.
 * Justified in Okami, where the keys in question are more like holy arrows, and the doors have a monster attached to them. Said monster can only be destroyed with the key/arrow, and the resulting explosion destroys the door. (Of course, the monster looks like a lock.)
 * Subverted in the The Matrix, there is a person who is called the Keymaster. He makes and keeps keys. In one scene, he has so many keys that it takes him a moment to find the right key to a important room.
 * The Tower of Druaga sort of justifies this by requiring a potion instead of a key to open locked chests.
 * Tibia has each key assigned to a specific door and not only that, the keys were reusable as well. You could freely lock doors you've just unlocked. In some cases, a key won't even be necessary to access an area because some other player may have left the door unlocked. It'll be risky to enter, though, as some other player may decide to lock you in with your only hope of escape being relying on the kindness of others, death, or waiting until someone else leaves the door unlocked.
 * Most Point and Click Escape-The-Room adventures will have multiple keys that are definitely not interchangable (since this would obviously lead to massive Sequence Breaking, such as using the first key you find to open the main door), but will disappear from your inventory as soon as you use them.
 * In Vampire Killer, each small key was only good for opening any one treasure chest, though you could sometimes uncover or purchase a lock pick that could open three treasure chests. You could only carry one of either at a time. You needed a different kind of key to open exit gates, but there was never a choice between those.
 * Mocked in Nerf Now here.
 * Mocked in a Cracked Photoplasty: Ads for Products That Must Exist in Video Games.