Lock and Key Puzzle

One of the two fundamental Adventure Game puzzle types. This sort of puzzle is based on collecting an item or items from somewhere in the model world, and delivering them to some other place, where they can be of use.

The archetype of this puzzle design is an actual physical lock. The player must retrieve the corresponding key from somewhere else in the world, and bring it to the door. Another very common form is a non-player character who performs a task which alters the state of the game world when he is given the proper payment.

In its simplest form, the player must do nothing more than bring the relevant object to the right place in order to solve the puzzle. Games with a more sophisticated user interface will require that the player use the "key" in some way specific to the nature of the puzzle, though in unskilled hands, this can lead to a Guess the Verb puzzle.

The Lock and Key Puzzle is one of the oldest and most common puzzle types across adventure games, especially Interactive Fiction. Its advantages include a tighter coupling with the model world than the Set Piece Puzzle. The setup is also easy to couple with plot developments, using the "key" as a Plot Coupon. The major disadvantage of this puzzle type is the possibility of Combinatorial Explosion.

See also Broken Bridge. If the "key" is something non-intuitive or ridiculous, you've got to Solve the Soup Cans.

Video Games

 * This one dates back to the dawn of Adventure: the goal of Colossal Cave is to deliver the various treasures in the game to a treasure room.
 * Zork has the same goal.
 * Ubiquitous in the Resident Evil series.
 * The ancient TinyMUD had a wonderful in-game language for constructing new puzzles. As long as they were lock-and-key puzzles. That was the only type you could build.
 * The two Detective Barbie PC games had this, although it wasn't always literal keys.
 * The NES game Castlequest was one gigantic version of this.
 * Several puzzles in the later Wizardry games ultimately boiled down to this, though quite often the nature of the item you need is only hinted at, and said item is hidden half a world away. Also common in reverse: you find the strange or seemingly useless item first, then much, much later, find the place you need that odd item.
 * In the first Megaman Legends game, you had to find starter keys hidden in dungeons in order to drop the shield that housed the refractor. In the second to last dungeon you had to find ID cards.
 * This was the closest thing there was to a puzzle in Doom; there were three different keys (red, blue, and yellow), each of which opened matching-color doors within the same level. Which keys appeared (if any) depended on the level.
 * Same with Quake and the gold/silver keys.
 * In Morrowind, there is an unnamed quest in the dungeons of Tel Fyr where the player would get a key and go looking through the tower and coprusarium for a series of chests that each contained some minor treasure and the next key in the line. Around the 8th chest, the treasures became very nice unique blunt weapons.
 * There are at least two puzzle like that in SpellForce. While not mandatory, they offer nice loot. They reappeared in the sequel, as well.
 * The more plot-driven Mystery Case Files games all end with variations of this.
 * Almost all dungeon levels in the Dark Cloud series feature this. The dungeon item will invariably be dropped by one enemy on the floor upon defeat, which will be used to travel to the next floor. Occasionally, you'll find keys that will open up doors or bridges that block off parts of levels.
 * Raocow refers to these as "item babysitting" in Super Mario World ROM hacks: finding a key and bringing it back to a lock you passed, finding a springboard and bringing it back to a high jump you passed, etc. He's not particularly fond of these puzzles, but it's about all the complexity and nonlinearity one can manage in the Super Mario World engine.
 * Eternal Darkness has many of these puzzles. One notable example is the "Staff of Ra"-style puzzle where the character finds a rod and a lens, puts them together and places the staff on a pedestal. The sunlight (or whatever it is) coming from above focuses through the lens into a beam, which must be rotated to reveal reflective panels before finally unlocking the door.