One Piece Wiki:Guidebook/Category Guidelines

Categories are lists containing pages that all fit into a certain group. They play an important role in site navigation, and thus must be handled with care and accuracy.

General Rules

 * Make sure to read the category page if you are unsure about placing a category on a page. Some categories have specific conditions that pages must meet before they can receive that category. For example:
 * Category:Children is for characters who ONLY appeared in the story as children.
 * Category:Former Pirates is only for people who stopped being pirates for reasons other than death or capture.
 * Category:Swordsmen is only for people who have actually used a sword; it is not for people who have simply been seen carrying swords.
 * Categories must be clearly defined. It should be easy to determine whether or not something meets the requirements to be in a category without being speculative.
 * Categories should note major traits. We should not have categories for minor traits, such as hair color, that are superficial to the story.
 * The name of a category should always be plural, unless it is the name of a group (such as Category:Worst Generation).
 * If your creation or addition of new categories is disputed, talk about it on the category talk page, do not edit war and add the categories back. Do note that even if a category does not violate any of the guidelines on this page, other users still have the right to question or dispute your proposed new category.
 * If you are thinking about performing a major category overhaul that impacts a large number of pages, proposing it on a forum or the category talk page is strongly recommended. Commencing disputed large-scale projects without input will result in a warning followed by potential blocks if these warnings are ignored, as these actions flood the wiki activity tracker and become a disruption if they are ill-thought out, on top of being extremely time-consuming to reverse.

Category Sizes

 * Try to avoid creating overly specific categories that only have very few pages in them, especially if those pages would fit in a more general category that already exists.
 * If you are creating a specific category from a more general category, there is a five page minimum both in terms of number of pages in the subcategory and number of pages remaining in the parent category (with the subcategory and pages in it not counting toward the latter total). Thus, if a category has eight pages, there is no reason to create a new subcategory with six of them, as all it does is make the web of categories unnecessarily larger and harder to navigate and splits up categories that were already small and easy to navigate. There is not a need to create comprehensive category tiers; such a task should mainly be reserved for truly large categories whose split would benefit navigation.
 * The five page rule does not apply to categories which cannot fit into a general category, but all categories must have multiple pages in them.

Matching Pages to Categories
A comprehensive guide for which categories belong in each type of article can be found in the Page Layout guide

Parent Categories
Parent Categories are categories that contain category pages. By doing this, anything categorized under a more specific subject can also be categorized under a general subject without having redundant categories. For example, everyone under Category:Straw Hat Pirates is a pirate, and so that category page is given Category:Pirates. Thus, by putting the Straw Hat Pirates category on a page, the subject will also be categorized under Pirates.

In order for a category page to be given a parent category, every page in that category must also belong in the parent category. Additionally, for the sake of navigation, we also require that parent categories must be directly related to the subcategory. For example, if you have a group or corporation that specializes in a specific occupation, then it would be appropriate to have that occupation as a parent category of the group category, as fundamentally anyone who is part of or will later join that group holds that occupation. However, if all the members of a category happen to share a trait that has nothing to do with their membership in the former category, then the category for that trait should not be a parent category. For example, if everyone in a pirate crew happens to all be male, but a female could join them at any time, then "Male Characters" should not be a parent category for that group's category.