Wikipedia:Do not use subpages

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One of Wikipedia's rules to consider:

Subpages are pages separated with a "/" (a forward slash) from the main page, e.g. "The Beatles/Penny Lane". The main advantage of subpages is that they make it easy to group related topics together without having to establish context for each of them. However, this is also one of their core problems. Subpages tend to get neglected and not follow style guidelines, and there are often far more subpages for a particular article than necessary. Many of the problems with subpages are specific to an encyclopedia, and subpages may work perfectly for other wikis.

Subpages were used on Wikipedia during most of 2001. A long debate then took place over whether they should or should not continue to be used. Larry Sanger, then editor of Wikipedia, took the position that they should be removed entirely. He wrote: "I think we've discussed subpages quite a bit--certainly enough to air the issues and give people a chance to state their views and change their minds--and in view of this, I've decided to get rid of them. (...) Finally, and probably as importantly as anything else, my well considered opinion is that the arguments in favor of getting rid of them are much, much stronger than the arguments in favor of keeping them. I predict yer gonna thank me in a year. (Maybe not all of you. :-) )"

His prediction turned out to be true, as subpages were then increasingly weeded out and replaced with redirects to standard article titles or deleted entirely. Many Wikipedians now have a strong aversion against subpages, so please do not be surprised if you see comments like "AAAHHHHHH SUBPAGE!!!! KILL KILL KILL" in the recent changes comments. However, due to inertia, some subpages still linger on Wikipedia, and new ones are occasionally created.

When you come across a subpage in the main article space, please move it to an appropriate title, e.g. "The Beatles/Penny Lane" to "Penny Lane" or "Penny Lane (Beatles)" if disambiguation is necessary. Do not forget to establish context. A page with hundreds of subpages (like the old Atlas Shrugged) may need more reorganization and summarization of different subpages into individual articles.

There are two exceptions for this rule. See Wikipedia:Subpages in the user and talk namespaces.

Arguments regarding subpages

The discussion below is primarily preserved here for historical purposes, as the decision against subpages in the article namespace is, for the most part, final.

Pro subpages

Ease of linking related data together

Helps link together related data: subpages can be used to divide an otherwise long article into sections; so can ordinary pages, but with subpages, the sections are connected automatically by being subpages. An unnamed online encyclopedia uses subpages for this purpose.
Subpages can also be used to facilitate linking to individual sections and between sections.
Subpages can also be used to create automatic links from the child to the parent and from a parent to the list of children; these links, appearing in a linkbar or other special place on a page, stand out and provide a useful, yet non-obtrusive, reminder to the reader of what "main" connections of the current page, in some useful sense of the word.
Can rename entire set of pages just by renaming main page.

Other advantages

Provides a useful home for data that wouldn't make sense on its own: subpages can be used to store small or large amounts of data about a subject that could be useful but would clutter the main page about that subject.
Similarly to the foregoing, subpages can be used to create small sub-articles that are puzzling as stand-alone encyclopedia articles, but which make sense qua encyclopedia articles as subpages of a main article
Established habit: they're known and used in the Wikipedia community, removing subpages might cause confusion among those who have used them and who have not practiced writing pages without them
Makes for concise titles: subpages convey the most information most concisely: for instance [[Algeria/Government]] vs. [[Government of Algeria]] or [[Algerian government]]
Useful for fictional universes and some other topics: subpages are particularly useful for collections of articles that have complex interrelations but very few if any relationships to topics outside the collection. For example Dungeons and Dragons and Lord of the Rings; perhaps poker.
Can be used to create standardised organisation of the same kind of relationship; for a trivial but by no means exhaustive example, consider "X/Childhood" in a biographical article versus competing schemes "Childhood of X" and "X's Childhood" creating confusion and unnecessary complication. (It seems however that all three schemes are equally arbitrary and one could standardize on either one.)
Autogenerating subpage lists: a nice feature would be to autogenerate a list of links to subpages on each page that has them. We haven't done this yet, but it would be nice. (It's in the PHP script; try [1])
Can be used to separate out meta-pages from the contents of the encyclopedia proper (superseded by namespaces)

Contra subpages

Hierarchy problems

Decisions on when or where to create subpages at all is necessarily arbitrary: every encyclopedia topic can be regarded as a subtopic of another encyclopedia topic. There is no good reason for us to regard some topics as subtopics of other topics when all encyclopedia topics can be so regarded.
The particular choice of a subpage hierarchy is arbitrary: [[Algeria/History]] might be used when [[History/Algeria]] would be as just as appropriate; both [[Film editing/Star wipe]] and [[Digital effects/Star wipe]] refer to the same thing and would be equally appropriate. There's no clear principles on which to make the decision, and the decision does have consequences.
Arbitrary subpage-imposed hierarchies arbitrarily contextualize information and thereby influence how articles are written: as one result of the foregoing, the small arbitrary hierarchy created by a parent page and its subpages quite often forces how we write content. Why should the people writing about star wipes be forced to consider them in the context of film editing as opposed to digital effects? If we write about the history of Algeria under Algeria/History, we'll consider Algeria's history as one element of Algeria's existence. If we write about the same subject under History/Algeria, we'll consider Algeria's history as one element of history. There is no good reason to impose this sort of constraint upon Wikipedia's writers, particularly when it is arbitrary. It simplifies the situation greatly to let each topic determine its own context, as it were.
Hard-wiring a conceptual scheme is imprecise; letting internal links do the work is more accurate: it seems much better to let links internal to the articles specify (in all the complex detail required) the conceptual relationships between articles, rather than "hard-wiring" them with a single very vague slash.
Single-level hierarchy is unjustified: subpages allow us to impose some conceptual structure or hierarchy on topics--but only one level of hierarchy. This is conceptually unappealing. Shouldn't it be either all or none? Look at it this way: suppose we create [[Foo/Bar]] and we're writing on the [[/Bar]] subpage. Then we discover a subsubtopic we want to create. What do we do then--just create a brand new top level page, apparently, for the subsubtopic. But then why do we have [[/Bar]] on a subpage in the first place? Doesn't make sense. This sort of thing could very easily happen in any area of human knowledge that is highly structured and "hierarchical," such as math, philosophy, and even history (there are many historical details of historical details).

Subpages replace the English meaning of the slash with a special meaning

The slash has no clear meaning and is therefore confusing in an article title: the slash creates a completely ambiguous relationship between the subject to the left of the slash and the subject to the right of the slash. For example, we could make "A" a subpage of "Countries of the world"; then, the list of pages under "A" would be the set of the countries of the world whose names in English begin with the letter "A." We could also make "Pearl Harbor" a subtopic of "World War II," and the relationship here is that Pearl Harbor is the-location-of-an-important-attack-in World War II. We could make "David Hume" a subtopic of "Philosopher" because Hume is a philosopher. Etc. Other punctuation has clear meaning. Wiki's slash does not. Therefore, it is better, for clarity, to eliminate the slash and replace it with English.
The slash has an ordinary meaning that subpages co-opt: giving the slash a special meaning within wiki co-opts its occasional ordinary use within English. Accordingly it sometimes creates "parent" pages that shouldn't exist, such as "8 1" in the title "8 1/2" or "GNU" in "GNU/Linux" or "Face" in the movie title "Face/Off".
Subpages are often written so as to require the contextualization of the main page, and new users often don't understand this: since the meaning of the slash in the context of this wiki is particularly unclear to new users, the meaning of a title of a subpage located by the search script or Google may be incomprehensible to the unsavvy user. In particular, they don't realize that the context is provided by the parent page, whose existence they are not aware of. They might not realize that the purpose or meaning of the subpage is given on the main page.

Other things to consider

The switch to the PHP wiki is a logical place to get rid of subpages. If we put it off, it will become increasingly difficult to do so.