Help:Category

A category is a software feature of MediaWiki. Categories provide automatic indexes that are useful as tables of contents. Together with links and templates they structure a project.

Putting an item in a category
A page in any namespace can be put in a category by adding a category tag to the page (by convention, at the end of the page), e.g.:

Category:Category name

This lists the page on the appropriate category page automatically and also provides a link at the bottom of the page to the category page, which is in the namespace "Category". Pages can be included in more than one category by adding multiple category tags. These links do not appear at the location where you inserted the tag, but at the page margin in a fixed place, depending on the skin (the bottom for Monobook, the upper right corner for Standard and Cologne Blue). Category tags may be placed anywhere in the article, although they are typically added to the end of the article to avoid undesirable text display side effects. Category links are displayed in the order they occur in the article, unlike the automatic ordering of lists in the category pages themselves (see below).

Summary
Each of the pages in the Category namespace represents a so-called category, a kind of grouping of related pages. For example, this page belongs to "Category:Glossary". When a page belongs to one or more categories, this information appears at the bottom of the page (or in the upper-right corner, depending on the skin being used).

The page of the category contains text that can be edited, like any other page, but when it displays, the last part of what is displayed is an automatically generated, alphabetical list of all pages in that category, in the form of links (in fact ASCII order, see Help:Special page).

New categories can be created and edited in the same way as any other regular page, but, as noted above, the displayed page contains more than what was in the edit box.

For a complete list of categories, see Special:Categories. Individual wikis may have their own top-level categories, such as w:Category:Browse in Central Wikia and many individual Wikia. For categories in Meta-Wiki, see m:Meta:Categories.

You can assign a category to a page simply by adding " Category:categoryname" to the page's wikitext source. Substitute the actual name of the category in place of categoryname.

To be specific, in order to add an article called "Albert Einstein" to the category "People", you would edit the article and add " Category:People" (no quotes) into its page source somewhere. Exactly where doesn't matter, but the Wikia policy, for example, is to put it after the article text, but before any interlanguage links.

In order to reference a category within a page as a normal wiki link (without adding the page to the category) prefix the link name with a colon. For example: :Category:Not in this category

Category page
A category page consists of any or all of the following:
 * editable text
 * list of subcategories; how many there are is also displayed; if there are no subcategories the header and count are not shown. With CategoryTree extension, installed on Wikimedia, one can click "+" to see the subcategories of a selected subcategory.
 * list of pages in the category, excluding subcategories and images; the number of items in this list is called the number of articles; if there are none the header is shown anyway, and "There are 0 articles in this category."
 * (From MediaWiki 1.5 onwards, the namespace prefix is no longer shown.)
 * list of images with thumbnails (how many there are is not counted); the first 20 characters of the image name are shown, with an ellipsis if that is not the full name; also the file size is shown. As opposed to the second and third section, this section does not have subheaders per letter, it has not even a header itself and visually seems to fall under the header of the previous section "Articles in category ...", even though in counting the articles, images/image pages are not considered articles).

The items in the lists all link to the pages concerned; in the case of the images this applies both to the image itself and to the text below it (the name of the image).

The first and second list each have a header for each first character, dispensed with if there are no entries for a header. If no headers are desired, use sortkeys all starting with a blank space (see also below).

On Meta and Wikipedia a redirect to a category shows the editable text only. To get the full page after being redirected, use the link "Image", or "Category", respectively.

On Commons a redirect to a category gives the full page, but not the subcategories and pages in the redirect page (if that is also a category).

Creating a category page
To create a category page, one can, e.g: or:
 * follow an automatically created link to the category page
 * add a colon in front of the Category tag when you set up the page-creation link, to prevent the software from thinking you merely want to add the page you are working from to the category:



Placing the above text on a working page will create the link you can use to edit your category page.

Sort order
The system uses alphabetical order, or more precisely Unicode order, for pages in categories. The range 32–127 corresponds to ASCII; for more, see a table of Unicode characters: !"#$%&'*+,-./0123456789:;<=>? @ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_ 'abcdefghijklmnopqrstuvwxyz{|}~&#127; Note in particular that "Z" comes before "a", and "z" before accented / umlauted characters. The blank space within a page name is treated as an underscore, and therefore comes after the capitals, and before the lower case letters. Thus we have the order PC, PCX, PC Bruno, PCjr. However, a "blank space" after the name comes before any character.


 * See also special characters, Pages starting with a special character, collating.

Sort key
Each of the three lists is in the order explained above. If you want an item in a list to be positioned in that order, based on an alternative name (sort key) for that item, then this can be specified in the category tag that places the item in the list:

sort key

For example to add an article called Albert Einstein to the category "people" and have the article sorted by "Einstein, Albert". You would type " ".

Although this is like the syntax for a piped link (and in an edit summary it is interpreted like one), there is a clear difference: the second term in the piped link is an alternative term for the first one, while the sort key is an alternative name for the page in which the tag occurs. Also, a piped link influences the rendering of the page itself, while a sort key affects the rendering of another page.

The displayed name is not changed, and is unfortunately visible only on the edit pages of the members of the category.

An explanation of the sort key system used can be useful on the editable part of a category page.

The sort key system should be obvious, otherwise the order seems random and items are hard to find. The system should either be consistently applied to all members of a category, or be such that the listings of members on which it is applied fit in well within the list of members for which no sort key is used. The latter is advisable for large, growing categories with many contributors. See also below.

A common system is starting with the last name, then a comma and a space, and then the first name; note that any additions such as "Jr." should come at the end, otherwise the entry would come before the same without the addition, because a space comes before a comma.

In e.g. Wikipedia:Category:Denmark, the sort key of a page consists of the keywords, without "Danish" or "of Denmark", "in Denmark", etc.

The sort key is case sensitive, so a page with the tag is sorted before one with the tag. This can be inconvenient, requiring a sortkey in projects with case-sensitivity of the first character if one wants the A and the a together, but it may also be used to one's advantage: e.g., it allows sorting of the pages into two groups, one put in the range A-Z and one in a-z, using sortkeys; see e.g. Wikipedia:Category:Demo, where the help pages are put separately by using sortkeys with lower-case "h".

To get the order right, be consistent in punctuation and spacing. For example, "A Z" with two blank spaces comes before "A A" with one. (Unlike double spacing in text, the double space is treated as is.)

An underscore in a page name is equivalent with, and treated as, a blank space. However, an underscore in a sort key is distinct from a blank space.

As seen from the ASCII sequence above, forcing items to the beginning or end can be done with a sort key starting with a space or ~, respectively. Another common sort key for the beginning of the list is the asterisk (*). Careful use of space and asterisk can produce two or more groups collected before "A".

Default sort key
[MW1.10+] It is possible to set a default sort key which is different from by using the magic word  thus: Once this has been included in an article, the new default sort key will remain in force until the end of the article, or until a fresh is used.

Sort key of images
Images without sort key are alphabetized according the full name, including namespace prefix "Image:". Therefore, for images for which a sort key is used, the prefix should also be included, otherwise these images are positioned on the category page before "Image:A" or after "Image:Z". Alternatively, all images can be given a sort key.

Thus a list of images may show a sequence A-I, A-Z, I-Z. One may choose to fix the first and third sequence by adding the prefix to the sort keys, or the second sequence, by putting sort keys without prefix.

Similar remarks apply for other non-main namespaces. However, the category namespace is an exception: the default sort key is without prefix, to avoid listing all subcategories by default under the C. Therefore, explicit sort keys should also be without prefix.

Grouping of pages by using sortkeys
If e.g. in a list of people sortkeys like "scientist Einstein, Albert" are used then all scientists are listed together under the "s". Unfortunately no separate heading "scientist(s)" is possible, and salesmen would be under the same heading. Alternatively "scientist" can have e.g. code K (to be explained in the editable part of the category page) and the sortkey "K Einstein, Albert" is used.

Labels in the list of images
As mentioned above, the first 20 characters of the image name are shown, with dots if that is not the full name. This can be inconvenient, e.g. in commons:Category:William-Adolphe Bouguereau most images have a name starting with "William-Adolphe Bouguereau (1825-1905) -", which are therefore all labeled "William-Adolphe Boug..." (see the last section of the category page).

The full names are only shown in the hover box and the status bar, depending on the browser; in the latter possibly with codes such as "%28" for "(".

Thus, looking up a name in the alphabetic list is cumbersome. It might have been better to start the names with the shorter "Bouguereau, " and then the title of the painting (unless the software will be improved later).

Using templates to populate categories
If a template contains the code indicating that it is in a category, this puts not only that template in the category, but also the pages that include the template.

The page that contains the template correctly lists the categories to which it belongs. However, adding or deleting a category tag in the template does not add or delete the listings on the category page of pages that use the template, until some edit is made in the page that uses the template.

In other words:


 * lists of categories a page is in, are up-to-date
 * lists of pages in a category are based on the situation just after the last edit of the pages

Since adding or removing a category or template tag is obviously an edit, there is only a complication when a page is indirectly added to or deleted from a category, through a change in a template the page uses.

To refresh category pages with respect to the listing of a particular page (adding or deleting the entry), a null edit can be applied to that page: just applying section edit and saving without changes.

Sort key
A sort key in a category tag in a template applies to the template as well as to the pages that use it, so it is only useful if it is variable. It can depend on a variable (notably PAGENAME), parameter, or the content of a template.

Categorizing templates themselves
To categorize templates themselves, without the pages that call them, one can use the tag, for example

Alternatively one can use e.g.

Excluding templates from categories
Use: to keep a template from showing up in a category. Text between and will be processed and displayed only when the page is being included. The obvious application is:


 * Adding all pages containing a given template to a category

Note that the usual update problems apply -- if you change the categories inside a template, the categories of the referring pages won't be updated until those pages are edited.

Comparison with "What links here"
Backlinks are often used as a by-product of links and inclusions. However, links and inclusion tags can be put specially for the backlinks, just as category tags are.

Thus one can create a kind of "category abc" showing its content with Special:Whatlinkshere/abc without an entry in the category lists on each page in the category. With inclusion this can be done in two ways:
 * rendering nothing. This works regardless of whether abc exists.
 * . This requires creating a blank page to render nothing, or a page with some content to provide an in-page category listing.

In the case of links:

A redirect corresponds to a supercategory.

Advantages of categories:
 * Category listings are alphabetical; for "What links here", alpha order typically applies for the first part only, for the pages already linking to the given page at the time of the last rebuilding of the link tables in the database.
 * Categories have an editable part (however, there is anyway a talk page)
 * A category can have multiple supercategories

Advantage of a pseudo-category system using backlinks: but not the contents of the sub-sub-subcategories (only up to three levels are shown); see Cat:Help and Cat:Municipalities of the Netherlands.
 * Backlinks can show a tree structure: not only pages and subcategories, but also:
 * the contents of the subcategories (for each a list of pages and a list of sub-subcategories) and
 * the contents of the sub-subcategories (for each a list of pages and a list of sub-sub-subcategories),

In the case of inclusions:
 * also multiple supercategories are possible
 * combined content of subcategories is shown in one list, not grouped by subcategory (the tree structure is not shown, but it can be used by taking the backlinks at a lower node, showing the smaller set)
 * the restriction of three levels (in the case of using redirects) does not apply; for example page Aijkl in the category Aijk in the category Aij in the category Ai in the category A (represented by Aijkl calling template Aijk, etc.) can be shown in the list of pages in category A (the backlinks of template A).

"What links here" tends to be a by-product of links that are useful anyway (although links may be put with this use in mind), while category links are put specially to produce a category page.

Putting a category tag on a test page, user page, etc. (also if done indirectly by including a template with a category tag) is considered to pollute the category (unless the category is specifically for material not in the main namespace), while regular links from such pages showing up in "what links here" are considered harmless.

Therefore:
 * when copying text to such pages, remove category tags; alternatively, convert the category to a link by adding a leading colon after the brackets. For example, "Category:Art" is a link to the category, not an assignment to a category.
 * when including a template that is in a category, do not use the template feature, but use "subst" or copy-and-paste, and delete the category tags.

Applying "Related Changes" to a category
For the "What links here" feature, only the links in the editable part of the page count, not the links to the pages in the category.

For a category, the "Related Changes" feature gives the changes in the pages in the category (according to the current category page, so excluding the pages that have potentially been added and including those that have potentially been removed, through addition or deletion of a template to/from the category, as explained above)
 * for subcategories: the changes in their editable parts only
 * for images (image pages): the changes in their editable parts only.

It does not show the changes in pages linked from the editable parts of the category. Possible workarounds:
 * The editable part can be put in a template. The category tags (which have no effect on Related Changes anyway) can better be kept out of the template, because on the pages of these supercategories the template would be listed under the articles in those categories. Whether interwiki links (with or without interlanguage link feature) are in the template or not does not make a difference, provided that the template is not used elsewhere.
 * Related Changes is applied to a page that calls the category as a template: ; only the editable part is included in the page; the page could be specially created for this purpose and call multiple categories. However, this page will then be listed in the supercategory pages of each category.

As usual (but as opposed to a watchlist) the changes in the corresponding talk pages are not shown.

Applying "Related Changes" to a category, with sufficiently high limits on number and days, is also useful for checking which pages in a category one "watches": they are bolded.

Dynamic page list
The DynamicPageList2 extension provides a list of last edits to the pages in a category, or optionally, just the list of pages; the simpler DynamicPageList is installed on Meta and Wikinews; the version DPLforum  is installed on Wikia, see  http://www.wikia.com/wiki/Help:Forums.

Detection of additions to a category
With "Related Changes" one can find pages which are newly in a category due to addition of a category tag or the tag of a template that is in the category. Addition of pages through addition to the category of a template is seen indirectly: one can see the change in the template, and then check which pages call the template. This even shows pages which are only potentially in the category (see above).

Unfortunately there is no similar way to detect a deletion from a category.

Category considerations
Each page is typically in at least one category. It may be in more, but it may be wise not to put a page in a category and also in a more general category.

Each category, except one top-level category, is typically in at least one higher level category.

Check the conventions in a project and make yourself familiar with the categories in use before assigning pages and subcategories to categories and before creating new categories.

Linking to a category
A colon (":") before the "Category" prefix, e.g., Category:NameOfCategory, will allow you to include a link to a category page without actually adding the article to that category.

Alternatives for overviews
An overview of links to pages and of images (by means of thumbnails) can be put in the editable part of a category, just as in any page, or be generated in the second, third, and fourth parts of the category page.

The first gives control on structure and lay-out. For example, in the editable part of commons:Category:William-Adolphe Bouguereau the name of the painter does not have to be repeated, and the full titles of the paintings can be shown.

The second is more convenient for new pages/images: by putting the category tag (which for images can even be done in the upload summary) the overview is automatically updated. Also, the sorting in alphabetic order is automatic in the second case.

A category page may have an overview in the first part, as well as in one or more of the other parts, with the following possibilities:
 * items occur twice, but ordered and structured differently, with different info (including that the fourth section shows the file sizes); when creating/uploading new items, they can be put in the category first, which is a convenient intermediate stage for updating the editable part
 * avoiding duplications

Anyway, a category has the double functionality of allowing extension of the overviews both from the overview page and from the member pages. This convenience makes it easier to build and maintain a complete overview (albeit divided over sections) for the subject concerned.

If the second, third, and fourth part of the category page are not used, then the category page is much like a page in another namespace. Differences are:
 * the name starts with "Category:"
 * in ordinary links to the category page the name is preceded by a colon
 * Related Changes does not work for the editable part

If a category is growing too large, it is cumbersome to split. When new items need to be added, and they are on a subtopic about which there is still little in the category, this is a good occasion to start a subcategory. Other kinds of overview can be split more easily.

See also commons:Commons:Images on normal pages or categories:Vote.

Subcategories
Creating subcategories takes only a few additional steps. Adding a category tag to a category page makes the edited category a subcategory of the category specified in the tag.

First create a new category page for the subcategory the same way you would make a regular category. For example, create.

Then go to the newly created category page and edit it. Add the category tag for the parent category (e.g. ) to the page.

In this example, the Soccer category would then be a subcategory of the Sports category. For a live example see Category:Demo_1 which is a subcategory of Wikipedia:Category:Demo.

Moving a category page
The only way to move a category page is to manually change all category tags that link to the category, and copy the editable part. There is no automatic way to move a category page in the way one moves an article page.

The editable first part of a category can be moved like any other page, but that won't move the subcategories, articles, and images in the second, third, and fourth part (see above). For categories entirely populated by templates (see above), modifying the templates allows one to move all affected articles to a renamed category.

Redirecting a category page is possible, but almost certainly won't have the desired effect (it can be abused for other purposes). In case other users try to re-create the category, leave the category in place but with Template:Category redirect on it - that directs users to where the page or subcategory should be.

List of all categories
Special:Categories provides an alphabetic list of all categories, with the number of members of each; this number does not include the content of the subcategories, but it includes the subcategories themselves, i.e., each counting as one. Comparing with http://meta.wikimedia.org/w/index.php?title=Special%3AAllpages&namespace=14 :
 * a category without members is not in the former
 * a category that has not been created as a page is not in the latter (in the former it stands out as a red link)

Singular or plural
It is not obvious whether a page like Amsterdam should be in category City (a description of a member of the category) or Cities (a description of the category as a set of pages). A convention for using one or the other is useful (e.g. on Wikipedia), to avoid ending up with both, with part of the applicable pages in one, and part in the other. A convenient rule of thumb is: if you can count some of the subject-matter, make the name plural.

Category flattening
Category flattening is displaying also the articles in a category's subcategories, sub-subcategories, etc.
 * category flattening with Semantic MediaWiki

See also Category flatten, and.

Category intersection and in-page display
Various MediaWiki extensions allow intersection of categories, and more:
 * intersection of unions of categories with Semantic MediaWiki - allows further intersection with sets of pages defined in terms of relations and attributes - can provide relation- and attribute-related info about the selected pages
 * intersection of categories and complements of categories with DynamicPageList
 * intersection of unions of categories and complements of categories with DynamicPageList2 - can also provide the time of last edit for each page
 * intersection of categories and complements of categories with Forum - can also provide the time of last edit for each page

They all also allow further restriction to a namespace; some allow also restriction to a union of namespaces, or the complement of a union of namespaces.

They all allow the info to be displayed in any page, not just a separate category page.

See also:
 * Wikipedia:Wikipedia:Category intersection

Category union
Various MediaWiki extensions allow union of categories, and more:
 * intersection of unions of categories with Semantic MediaWiki
 * intersection of unions of categories and complements of categories with DynamicPageList2

See also above.

Display of info about the pages in a category
Various MediaWiki extensions allow display of info about the pages in a category, and more:
 * providing relation- and attribute-related info about the pages in a category with Semantic MediaWiki
 * provide the time of last edit for each page in a category with DynamicPageList2
 * provide the time of last edit for each page in a category with Forum

See also above.

In-page display of categories a page is in

 * in-page display on any page of categories the same or another page is in, with Semantic MediaWiki