Key items, also known as Important Things in Paper Mario: The Thousand-Year Door and Super Paper Mario and Useful Items in Paper Mario: The Origami King, are a category of item that appears in many role-playing games, including those in the Super Mario franchise. Key items are typically distinguished from ordinary items by being unique and one-of-a-kind, though in some cases, a finite number of copies may be obtained, such as in the case of the four Fortress Keys in Paper Mario. They are usually necessary to progress in the main storyline of a particular game or otherwise in some sort of sidequest, and as such, they cannot be as easily consumed or discarded as other items, except when doing so is necessary to progress. Similarly, in games with limited inventory space, such as Paper Mario, key items usually do not use up the inventory slots that are used for other items.
Key items are sometimes distinguished from other kinds of finite, non-consumable items, such as badges and clothing, which may be given their own categories on inventory screens and other similar treatment.