Help:Page name

The following characters are not allowed in page titles:
" # $ * + < > = @ [ ] \ ^ ` { } | ~
The reasons include::* @ also has a special meaning in URLs, as a way of adding a username and password, and would have even more drastic consequences.:* $, \, ", ` (and some others) have special meaning in other bits of the software, and allowing them would create potential security flaws which would take a lot of effort to insure against.
There are some pages not satisfying the restrictions, e.g. w:$. They may give complications.
Some very special characters, like two dots over the n that has been attempted for the page w:Spinal Tap, are not allowed either. They can only be represented using Unicode, whereas the English Wikipedia just uses w:ISO 8859-1 or similar.

Also, the first part of a page name may not coincide with a project-independent namespace prefix that is automatically converted to another one, e.g. the name Project: A-Kon on Wikipedia is not possible.
The first part of a page name can coincide with a namespace prefix that is not converted. For example, there might be articles in the English Wikipedia about books called Wikipedia: The Big Adventure and Talk: Secrets are Bad (but only without the space after the colon). However, in that case the pages are in the wrong namespace. This may be inconvenient in searching or displaying a list of pages. Also, in the second case there is no link to a Talk page about the book. (As explained above, the second page name is not possible on e.g. the German Wikipedia: see w:de:Talk: Secrets are Bad).

A page name can not start with a prefix that is in use to refer to another project, including language codes, e.g. "en:" (list), or one of the pseudo-namespaces "Media:" and "Special".

In page names, a blank space is equivalent with an underscore. A blank space is displayed in the large font title at the top of the page, the URLs show an underscore. See also below.

If for the first letter of a page name two cases exist, as in the case of letters of the Latin, Greek, Cyrillic, and Armenian alphabets, the following applies.
All characters of namespace prefixes are case-insensitive. The canonical form, shown in large font as page header, and in URLs generated by the system, is with one capital. Below "page name" refers to the name without the possible namespace prefix.

The first character of the page name may or may not be case-sensitive, depending on the project. [[Help:page name]] gives on this project: page name. If the first character of the page name is case-sensitive this is a link (to a different page), otherwise it is bold (a self link to this page).
Currently in all Wikimedia projects other than the Klingon Wikipedia and the toki pona Wikipedia it is not. For these two, compare e.g. w:tlh:jo and w:tlh:Jo, and w:tokipona:musi with w:tokipona:Musi. For Wiktionary changing this is being discussed at Wiktionary:Wiktionary:Beer parlour/case-sensitivity vote.

The canonical form is with a capital.
A link like [[template]] works like a piped link [[Template|template]]: [[template]]; unlike a redirect, the conversion shows up already on the referring page when pointing at it: in the pop-up and in the status bar (if applicable for the browser).
Note that in the case of a prefix that is not a namespace for the software, and in the case of a second prefix, the case-insensitivity does not apply to the first character after this prefix, e.g. H:Interwiki linking and H:interwiki linking are distinguished.

Note that even the file name extension of an image is case-sensitive: compare Stop_sign_us.jpg and Stop_sign_us.JPG

Spaces/underscores which are ignored:
[list][*]those at the start and end of a full page name
[*]those at the end of a namespace prefix, before the colon
[*]those after the colon of the namespace prefix
[*]duplicate consecutive spaces
[/redirect]
Some show up in the link label, e.g. [[___help__ :_ _template_ _]] becomes ___help__ :_ _template_ _, linking to Template.
However, a space before or after a "normal" colon makes a difference, e.g. MediaWiki User's Guide: Editing overview and MediaWiki User's Guide : Editing overview, and MediaWiki User's Guide:Editing overview are all distinguished, because "MediaWiki User's Guide:" is a pseudo-namespace, not a real one.

A page name can not contain e.g. %41, because that is automatically converted to the character A, for which %41 is the code. [[%41]] is rendered as %41. Similarly %C3%80 is automatically converted to the character À. [[%C3%80]] is rendered as %C3%80. The URL of the page is http://meta.wikipedia.org/wiki/%C3%80. One can argue what is the real name of the page, %C3%80 or À (a user will say the latter), but anyway there can not be distinct pages with these names.

Alphabetical order, e.g. in Allpages, is (at least for that range) according to ASCII. Note that this means that "a" comes after "Z", see e.g. [1].

Variable {{PAGENAME}} gives for this page Page name, Variable {{PAGENAMEE}} gives Template:pagenamee.
Thus in the first case a space is used, in the second case an underscore, like in URLs. Similarly À becomes the escape code %C3%80 (see above), etc.
{{NAMESPACE}}:{{PAGENAME}} and {{NAMESPACE}}:{{PAGENAMEE}} give for this page Help:Page name and Help:Template:pagenamee, respectively. For a page in the main namespace the page name is prefixed with a colon.
Example:
http://meta.wikimedia.org/w/wiki.phtml?title=Special:Whatlinkshere&target={{NA MESPACE}}:{{PAGENAMEE}}
gives
http://meta.wikimedia.org/w/wiki.phtml?title=Special:Whatlinkshere&target=Help:Template:pagenamee
{{PAGENAME}} would not work.
With a Google search there is the problem that for Google the space and the underscore are different, see w:Template talk:Google.

[list][*]Link gives examples of conversions in the page name from wikitext to HTML, and from HTML to actual target page.
[*]MediaWiki_1.3_comments_and_bug_reports#Comments_re lated_to_page_title
[*]Proposed_Wikipedia_policy_on_foreign_characters#Ar ticle Titles (old)
[*]w:Wikipedia:Naming conventions
[*]w:Template:Wrongtitle
[*]w:Wikipedia:List of pages whose correct title is not allowed by MediaWiki
[*]Case insensitivity - about a proposal for case-insensitivity of all characters in the page name.
[*]http://cvs.sourceforge.net/viewcvs.py/wikipedia/phase3/docs/title.doc?rev=1.1.1.1&view=markup - not quite accurate: [[:de:name]] does not give a page with the name de:name.
[/redirect]Media:Example.ogg