Key items are a recurring element of the Resident Evil series and are usually found in RPGs. These items are required for the completion of puzzles or the fixing of machines, and aide in the progression of the story. Key items greatly differ from normal items such as ammunition, money, or healing items since in most cases you can only obtain one in the entire game.
Despite the name of the term, key items are not always keys. They have ranged from broken weapons to gemstones, tools to stone tablets, and so on - some Capcom-Famitsu guidebooks refer to these as "Event items".
Recurring items include actual keys, but also Valve Handles and Cranks. In some instances, personal items used by player characters become necessary to solve puzzles, exchanged for key items, or necessary to use key items. In many cases, a personal item not held by the opposite player character needs to be picked up by the other somewhere else to progress. The Lighter is a common example of all four of the above qualities, having been used in all of these ways throughout the series.
There are key items that can be stacked, such as the Small Key item, that are used in place of the Lockpick to unlock doors for the male characters. Other items simply have more than one piece that need to be collected to open a door. In rare cases, such as the Virgin Heart in Resident Evil 2, there are two of the same key item necessary to progress, but they do not stack in the player's inventory.
In older titles, key items take up one space of the player's inventory. In rare cases, some take up two. The Hookshot from Resident Evil 0 is an example of this. Starting in Resident Evil 4, key items take up no spaces in the inventory at all, and were simply kept in a separate menu. Resident Evil 5 shows which key items the player has at the top of the screen. Older games allowed the player to refuse picking up the item and also store them in the Item Box. Resident Evil: Revelations 2 make a comeback that key items take up space in the inventory again.
There are some key items that are optional, and only used to access extra ammo or weapon upgrades.