Wikipedia:Why do we have outlines in addition to...?

See also Rationale for outlines.

Occasionally, an editor will notice that outlines sometimes overlap in scope and function with another type of page, and wonder why we have both. In general, it is because outlines are optimized for browsing and for showing a subject's structure. And because readers vary in their learning styles: some readers find outlines more convenient for exploring and for review than other forms of knowledge access and presentation. For more information about redundancy between Wikipedia's info navigation systems, and its benefits, see WP:CLN.

Getting back to the question, Why do we have outlines in addition to...

...subject articles?

Outlines are a form of list, and lists are easier and faster to browse than the text (prose) in articles. Links take longer to identify and click on in articles (they're spread out more). Articles are intended for explaining, while outlines are a type of tree structure optimized for navigating and depicting taxonomic information rapidly. Outlines are more effective at covering the entire scope of a subject, and outlines are not limited by the size restrictions that the see also sections of articles are. It is awkward to present as many links in an article, and embedded links tend to be arranged much more chaotically or even randomly throughout an article's prose. The links in an outline are much more organized for ease of understanding.

In addition to being a type of article, outlines are also an important part of Wikipedia's contents system, and serve as tables of contents or site maps for their respective subjects.

...portals?

Over the years, a few users have suggested that outlines and other topic lists be moved to portal space and added to the corresponding portals, or combined into a single portal with outline subpages. This would be generally undesirable, because articles, including stand-alone lists, have greater utility than portal subpages. And outlines are a type of stand-alone list. More specific reasons are that...

Portals are harder to create, develop, and maintain. Portals can't keep up with the number of outlines being produced. There are already many more outlines than there are portals (currently, not all of them are named "outline of"), and as time goes on, there will be increasingly more outlines than portals.

Pages in article space (including outlines) show up in Wikipedia's search results by default, portal subpages do not. And when portal space is added to search results via "my options", they make all search results almost unreadable. More importantly, since most outlines cover core subjects, it is imperative that they remain searchable.

There are fewer restrictions to linking to articles, and the simplicity of article titles (including outline titles) make them easier to work with than portal subpage titles.

As a type of article, outlines are designed to cover the entire scope of their subjects, while portals are modeled after the Main page and are designed to sample a subject by presenting excerpts of a small selection of articles. Outlines are for browsing an entire subject, while portals are for presenting suggested reading similar to the Reader's Digest.

Being more comprehensive, outlines tend to be larger than portal pages, and wouldn't readily fit into most portal's designs without making the portals overly long, cumbersome, or unbalanced.

Some portals have their own topic lists, but the scope of their lists is very limited compared to that of outlines, and the complex table formatting of the lists on portals make them much harder to create and develop – even more so than the other parts of a portal. And converting existing outlines to these formats would be extremely tedious and time-consuming.

Outline formats are standardized, making outlines very useful for comparing similar subjects (such as comparing countries).

...categories?

Like other types of lists on Wikipedia, outlines should never be deleted in favor of categories or navigation templates. For more information see Wikipedia:Categories, lists, and navigation templates.

Outlines have many advantages over categories, which they complement rather than compete with...

Outlines are articles, and show up in Wikipedia's search results by default. The lists in categories are invisible to searches – only the category pages themselves are accessible to searching, but not by default.

Outlines show multiple levels on the same page, and support scrolling for ease of scanning with the eyes – once loaded, an outline is faster to browse than a branch in the category system (which relies heavily on clicking and subjects the reader to intermittent server delays, especially when the reader needs to traverse back and forth up and down the tree).

Outlines can list topics that don't exist as articles yet, and help to direct expansion of a subject's coverage.

Outlines support redlinks, which can be named based on the pattern expected for the expansion of the subject's coverage. Redlinks can be clicked on to create new articles, or to create redirects to the current location for the material on their topics. When an article is deleted, it vanishes from its categories. On outlines, its link would turn red which alerts editors of the problem.

Outlines support redirects. Presenting projected article names as section redirects has the advantage of their being converted to actual articles later as the subject expands, while reducing the need to have links updated (because the updating takes place on the redirect pages themselves). Redirects are generally not included in categories (see WP:CAT-R).

Outlines support reference citations. Category entries can't include citations.

Outlines are supported by edit histories, so changes to an outline can be reviewed. Categories don't have such support, so entries can disappear without their vanishing being noticed.

Outlines can be watchlisted. Category entries are invisible to watching.

Outlines support all the advantageous features of lists, including annotations (such as descriptions, explanations, and related information), pictures throughout, headings, subheadings, section leads, templates, etc. These features cannot be interspersed throughout the links listed within a category.

Because outlines can be edited directly, outlines are much easier and faster to create, develop, and modify than categories. Outlines can also be modified offline for further convenience. Editing categories entails editing a separate article for each entry in a category to be added, moved, or removed. There is a server delay after every edit to a category's list of links. An outline can be edited extensively and then saved.

Outlines are extremely customizable and can accommodate almost any presentation need.

...indexes?

Indexes on Wikipedia generally present articles in alphabetical order. Outlines present them as a hierarchy, showing the "familial" (parent-offspring-sibling) relationships between them, which is useful for finding and browsing subjects closely related to each other.

Indexes are comprehensive in scope and are intended to include all of the topics on a subject. Outlines by definition should include only essential links, and because of this they don't get watered down by several hundred types of cuisine or 10,000 person or place names – links to lists of these suffice in outlines, which makes outlines easier to browse.

For example, the article index for Japan has grown to be 23 pages long:

Alphabetical index: 0–9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

But if you want to see at a glance the structure of Japan as a subject, you might find the Outline of Japan more useful.

The difference between an outline and index on Wikipedia is the same as the difference between the table of contents and index of a book.