From Wikimedia Commons, the free media repository

A category is a software feature of MediaWiki, a special page which is intended to group related pages and media. In practice, it implies that you'll associate a single subject with a given category. The category name would be enough to guess the subject, but some extra text can be useful to precisely define it.

[edit] Categories in Wikimedia Commons

The category structure is the primary way to organize and find files on the Commons. It is essential that every file can be found by browsing the category structure. To allow this, each file must be put into a category directly, and/or put on a gallery page which is categorized. Each category should itself be in more general categories, forming a hierarchical structure.

[edit] Categories and/or galleries?

Note the "and/or" above. Some users are of the opinion that when an image is included in a gallery article, it is sufficient to categorize that article. Others believe that each individual image should be categorized too. The consensus on this issue, as determined by vote, is that both systems are equally valid and should be used concurrently. Do not remove categories just because an image is in a categorized gallery. It is hoped that in the future the software will be updated to merge the functionalities of galleries and categories into one. Automated tools for finding images must dig through categories and gallery articles.

[edit] Category structure in Wikimedia Commons

Don't place an item into a category and its parent (e.g., Put it in Chocolates, not Chocolates and Sweets)

The category structure is (ideally) a multi-hierarchy with a single root category, Category:CommonsRoot. All categories (except CommonsRoot) should be contained in at least one other category, and there should be no cycles (i.e. a category should not contain itself, directly or indirectly). The category structure should reflect a hierarchy of concepts, from the most generic one down to the very specific.

More detailed see Meta:Help:Category#Category structure

[edit] Major categories

The top-most categories (the ones contained directly in CommonsRoot) divide the category structure by the purpose of the contained categories:

• Category:Commons - This category is the global common root of categorizing Commons' maintenance tasks and pages (Commons:-, and Help:-) except for media files. The translated pages in each language should be categorized under the each language category such as "Category:Commons-ISO-LANGUAGE-CODE" style. The structure of Category:Commons-en is the sample hierarchy for every other language sub category.

There is a sub category Category:Commons maintenance content, which is for the special maintenance of Wikimedia Commons' global common contents and which does not get translated.　ALL media files should be categorized under the first 4 categories below, but ONLY files having problems and needing to be fixed should also be in the sub-category Category:Commons maintenance content.

• Category:Image sources - This category is the global common root of the media files categorized by the SOURCE, where they come from (books, collections, sites, etc.)
• Category:Copyright statuses - This category is the global common root of the media files categorized by the LICENSE. ALL media files should be categorized under this category with the appropriate license tag.
• Category:Media types - This category is the global common root of the media files categorized by the Media TYPE. Please note that this type of categorization is sometimes omitted for images, since the vast majority of files on the commons are images of some sort.
• Category:Topics - This category is the global common root of the media files categorized by the TOPIC. ALL media files should be categorized under this category for the sake of allowing others to find them by topic.
• Category:User - this is for categories that contain commons users, sorted by things like the language they speak. This also contains the Category:User galleries, which is for user specific (i.e. non-topic) galleries.

[edit] How to use categories

You should always put your uploads into categories and/or gallery pages according to topic, so your contributions can be found and used by others.

It is rarely necessary to create a new category (but for an exception, see People below). Before doing so, make sure you are familiar with the existing category structure, and with the customs and policies of the Commons. Please see if there exists a category scheme or a commons project for your topic, and follow the conventions described there.

Please provide as much details about your uploads as possible — see Template:Information for a suggestion on what to include. You must also provide license information for your uploads — see Commons:Licensing for details. See Commons:Project scope for information about what media are suitable for Commons.

[edit] Category name

Category names should always be in English (except life forms, for which the scientific Latin name should be used). Category names that refer to objects or groups of people should generally be in plural form: Category:Tools, Category:Artists, Category:Lakes, etc, as opposed to Category:History, Category:Weather, Category:Music. See Commons:Naming categories for more information.

We still lack internationalization for category names, but this issue should be resolved with appropriate changes to the MediaWiki software (see bugzilla:5638). Creating intermingled category structures in different languages would only make things worse.

For a general discussion of MediaWiki's category feature, see the manual page on categories.

[edit] Categorizing pages

To add a page (be it an image, a gallery page, or a category page) to a category, add the following code to the end of the page.

[[Category:Category Name]]

For example, if you are uploading a diagram showing the orbit of comets, you could add the following to the image description page:

[[Category:Astronomy diagrams]]
[[Category:Comets]]

This will make the diagram show up in the categories Astronomy diagrams, Comets.

For information on how to find good categories for your uploads and galleries, read the section Find an appropriate category below.

[edit] Creating a new category

To create a new category:

1. Do a thorough search, to be sure there isn't an existing category that will serve the purpose.
2. Find a gallery page or other page which should be put in the new category. Edit this page, and at the end insert the new category reference. e.g. [[Category: Title]]. Save the edited page. The new category appears as a red link at the bottom of the page.
3. Click on that red link. The new, empty, category page appears for editing. You can now edit the category like any other wiki page.

A category page should contain the following information (in order of importance):

• Category-links that put it into one or more parent categories.
• A short description text that explains what should be in the category. English is the preferred language for the description, other languages can be added (with the template {{ab|...}} for description in Abkhazian, {{en|...}} for description in English, etc, as listed in Commons:Templates for galleries).
• Interwiki links to categories or pages with the same topic in the Wikipedias (i.e. interwiki link [[ab:...]] to the page in Abkhazian Wikipedia, [[en:...]] to the page in English Wikipedia, etc.).
• If the category should be sorted according to a different string than the category title, add a DEFAULTSORT template. For instance, the title of a category about a person would not be the right sort string. For such categories, insert after the interwiki links a line like {{DEFAULTSORT:Lastname, firstname}} with the correct sort string.
• Put the new page in any related categories is should be in, according to the guidelines. At bottom of the new page, insert lines of the form [[Category:Relevant categories]].

See also #How to categorize: guidance by topic for guidance on specific classes of category, e.g. categories about #People.

[edit] Renaming or moving categories

Please see Commons:Rename a category.

[edit] For more appropriate categorization

Pages (including category pages) are categorized according to their subject, and not to their contents, because the contents is generally not a permanent feature of the category page; in particular, you can momentarily find inappropriate contents in a category page.

Example: Assume that Category:Spheres contains only pictures of crystal balls. You must not add Category:Glass in the category page, according to the current contents, because you can have spheres made with a great variety of materials. Normally, any picture showing a glass object would be already categorized in Category:Glass (or in a category of its substructure). So, if the Category:Spheres is really crowded with crystal balls pictures, it would be a better idea to create a new category page, like Category:Glass spheres or Category:Crystal balls, categorized in Category:Spheres and Category:Glass.

Generally files should only be in the most specific category that exists for certain topic. For example files in Category:Paris should not also be in Category:France (see over-categorization below). If you do not find a category that fits your purpose, you can create it — but carefully read the section about using categories first.

This does not mean that an image only belongs in one category; it just means that images should not be in redundant or non-specific categories. For instance, an image of a Polar Bear being rescued from an iceberg by a helicopter should be in Category:Ursus maritimus, Category:Icebergs, Category:Helicopters, and Category:Search and Rescue. It should not, however, be in Category:Ursidae or Category:Aircraft.

[edit] Categorization tips

The categories (or galleries) you choose for your uploads should answer at least one of the following questions:

• what? / whom?: what or whom does the file show? For instance Category:Houses or Category:Jimmy Wales
• where?: where was the image taken? This is especially important for pictures of places, obviously. E.g. Category:Saxony
• when?: this should provide some historical context, if relevant. An example would be Category:World War II
• how?: this should give some information about the media type, like Category:Audio
• who?: if the artist is well known, like Category:Rembrandt, it makes sense to categorize by artist; you can also use the pages from the Creator namespace as templates to achieve this. See Commons:Creator for details.

[edit] Find an appropriate category

To find appropriate categories for your uploads, you should navigate the category structure starting from a generic category. Narrow your search down to subcategories until you find the most specific category that fits the file you uploaded. You can navigate the category structure by following links to subcategories, or expanding the tree of subcategories by clicking on the little + symbols on subcategory names. The Major categories section above provides a starting point, and the How to categorize: guidance by topic covers some topics more. You can also try CommonSense, a tool that is designed to help with categorization based on keywords.

[edit] Over-categorization

Over-categorization is what happens when an image is placed in several categories within the same tree. The general rule is always place an image in the most specific categories, and not in the levels above those. An example:

We'll assume that yellow spheres are spheres with a yellow color. We can think about Category:Yellow spheres and Category:Spheres. The picture to be categorized shows yellow marbles. We categorize the file in Category:Yellow spheres. Now, if we also categorize the image file in Category:Spheres, this is over-categorization: because we already know that the yellow marbles are spheres. This applies to most images: As mentioned above files in Category:Paris should not also be in Category:France, files in Category:Albert Einstein should not be in Category:Physicists from Germany and so on.

Visually, it is the same problem as the red arrow shown in the chapter above.

[edit] Why is over-categorization a problem

It's often assumed that the more categories an image is in, the easier it will be to find it. Another example: By that logic, every image showing a man should be in Category:Men, because even if you know nothing more about the person you're looking for than that he is a man, you'll be able to find it. The result is that the top category fills up, making it necessary to go through hundreds, or in this case more likely thousands of images to find the one you want. You probably won't find what you're looking for, and what's more, those who are looking for a generic picture of a man to illustrate an article like en:Man will find that they've drowned out among the movie stars, scientists and politicians.

On lower levels, the problem becomes less acute, since the number of images will be smaller — they can still easily reach into the hundreds, though. But there is still a problem: Let's go back to Einstein. I know that he's a physicist, so I'll look there. I find an image among the hundreds in the category, which I'm not too happy with, but it's the only one there. Since there was an image there, I assume that there are no more hidden elsewhere, rather than look further in Category:Physicists from Germany and thus find Category:Albert Einstein where there might be a better one. So over-categorization has led to two problems: The top category is cluttered, and users will stop looking for the most relevant category since they've reached one that has a relevant image.

[edit] Improper categorization of categories is a cause of over-categorization

Strange as it may sound, under-categorization can actually be a cause of over-categorization. This happens when a category is not properly categorized, leading users to over-categorize an image to get it into the relevant categories. An example of this: Category:Eivør Pálsdóttir was categorized only in Category:People by alphabet. So if I add an image of her, and know who she is, I would also place the image in Category:People of the Faroe Islands and Category:Vocalists. This is over-categorizing, I've caused clutter in the top categories by adding images directly to them.

A related problem is erroneous categorization: Category:Notting Hill was for more than a month placed in Category:London. When adding an image, it would be very tempting to add that image to Category:Kensington and Chelsea, which is where you'll find Notting Hill. Instead, each image should be placed only in the most specific categories, and those categories should in turn be placed in its most specific categories.

When you encounter this, please categorize the categories properly if you are able to do so. That will not only help avoid over-categorization, but also make it easier to move through the category tree.

[edit] How to categorize: guidance by topic

For some categories, there is special guidance on how best to sort content within that category. Some of that guidance is in this section. There is also sometimes guidance at the top of the category's page, in the Category namespace. So, for instance, some guidance on categorizing content depicting people is at the top of Category:People, and some is in the section People below.

[edit] People

Content depicting people can be put in categories and/or galleries which describe them, such as Category:Economists from the United States. Start exploring at Category:People.

Some notable people may have so much content depicting them, or names that are so well known, that it may be worth creating a category or gallery using their name, such as Category:Albert Einstein. (This is one case where you may find yourself creating categories frequently, i.e. each time you upload the first picture of a notable person.) Use the common English-language spelling and word order for the category or gallery name; it's a pity to force this on all the world's names, but as noted elsewhere, we use English-language categories on Commons to work around a software limitation.

Some categories are descriptive of persons, e.g. "Category:Physicists from Germany". Where there is a category or gallery named after a person, don't put such descriptive categories into individual image pages showing that person. Always put them instead in the person's category or gallery page. (Why? Because descriptive categories apply to all images of a person, and this rule achieves that with just one descriptive category entry.) So, the only category entries in an individual image page should be a link to the category or gallery page for each person depicted, and maybe categories that distinguish this image of that person from other such images.

Put into each gallery or category named after a person, the category Category:People by alphabet and fill in their family name: e.g. [[Category:People by alphabet|Family name, first name]]. Again, do not tag image description pages with this category! (Why? There are so many image articles of notable people, that including them directly would overwhelm the category. Limiting Category:People by alphabet to category and gallery pages greatly reduces the number of entries.)

Insert a line like {{DEFAULTSORT:Lastname, firstname}} towards the bottom of the page, just above the related categories. This line gives a string to use for sorting, instead of the Category's title. Usually, the order that results from sorting by category title isn't the correct order for a list of peoples' names.

[edit] See also

• Commons:Category scheme Commons maintenance/en
• Commons:Categories/redirect bookmarklets
• Commons:Deletion guidelines#Categories for deleting/redirecting categories
• {{Uncategorized}}
• Commons:Galleries