Editors should generally follow it, though exceptions may apply. Substantive edits to this page should reflect consensus. When in doubt, discuss first on the talk page.
This page in a nutshell:Substitution is the one-time, permanent copying of template content on Wikipedia. It differs from transclusion, which continually updates as the linked-to content changes.
Substitution is a different way of handling templates than normally done on Wikipedia. Usually, templates are transcluded, which means that if the template changes, every page that uses it will update to comply with the change-in-question. This is the procedure used for most templates. However, some templates are instead substituted. This means that if the template changes, the page that uses it will not, and will continue to show the old version of the template. Some templates should always be substituted, while most should always be transcluded.
To substitute, add subst: after the opening braces, as in {{subst:page name}}. For example: substitution{{subst:Medicine}} in contrast to transclusion{{Medicine}}. The contents of Template:Medicine will be expanded (for any templates) and stored at each point where "{{subst:Medicine}}" is placed in a page. Note that ref tags refuse to run "subst:" unless temporarily renamed as "<xref name=xx>" or similar (see the bug report at T4700).
This page also lists templates that should either always or never be substituted except in the Wikipedia namespace. Automated tools (a.k.a. bots) that do such replacements will never be used on the original template. If you are going to use a bot to substitute templates, please read through the talk page first, as many are under dispute or change status over time, and substitution is permanent.
About subst:
Definition
The subst: keyword (short for "substitution") is used inside template code, placed as a prefix before the name of the template but within the curly braces ({{}}). It changes the way the MediaWiki software expands the template.
Ordinarily, a template will be expanded "on the fly"; that is, the template code on a page calls a separate page every time it is rendered. Although most page views are served from the cache, pages need to be rendered for previews and rendered again when the page changes. When someone is editing a page with a normal template call, they see the template call.
Placing "subst:" inside the curly braces tells the software to permanently replace the template with the text of the template (i.e., the text that is on the template's article page when the template is added to the page). Therefore, {{template}} becomes {{subst:template}}, until you save the page. The next editor sees not the template call, but instead the text of the template when you saved; it does not change even if the original template is edited.
Usage
You can substitute a template tag by adding "subst:" to it. For example, use {{subst:test5}} instead of {{test5}}.
When this is a major component of an edit, it is strongly suggested that the template be mentioned in the edit summary (for instance, put "{{subst:test}}", "subst:test" or "test1 applied" in your edit summary) so other editors can easily see what you have done.
Additionally, when creating or editing templates that are commonly substituted, adding a hidden comment to the template page helps article editors to see how templates are being used (e.g. <!-- Substituted from Template:Documentation -->). Use {{subst:html comment}} if you want to write HTML comments that contain wikitext expansions.
Benefits
If there is a template you would like to modify for a single occasion, but you do not want to modify the template for all the pages that use it, and you do not want to make a new template, you can substitute the template and then with a second edit, make the modifications. For example, a template might be used with all the states of the United States and with the District of Columbia. The template might be worded correctly for the states, but not for DC. To fix the wording, you can substitute the template and then fix the wording.
Substituting frequently used, but unprotected templates, limits the damage that can be caused by people vandalizing the template. See high-risk templates.
Transcluding certain templates hides wikitext from newcomers, whereas substituting makes the wikitext visible and gives them the opportunity to learn its use (of course, there is a "how to edit" link at the bottom of each edit page).
Substitution allows for recursive evaluation through macro templates.
Neutral
Substituting en massemay ultimately speed up the site, but this is not a reason to prefer substitution over transclusion. Don't worry about performance of Wikipedia's servers.
Templates are often modified or deleted. If a template is boilerplate text, consider whether you want it to vary as the template is modified. If your answer is "no", substitution is warranted. An example of this is the {{subst:welcome}} template. The archives of a user's talk page should show the actual welcome message they received, not the current welcome message.
Drawbacks
Once a template is substituted, the result is no longer linked to the template, making it hard to find all pages displaying that text (though categories can sometimes relieve this). The problem can easily be worked around by including a link to the template in the template's code.
A substituted template will not be updated when errors in the master template are corrected.
If the template is used to standardize the appearance of something, you probably do not want to do a substitution. An example of this is a table of contents or navigation box.
Substituting en masse—editing thousands of articles with bots—temporarily slows down the site and wastes server resources unnecessarily.
Substitution increases the size of articles in the database and database dumps.
A substituted template can add a lot of wiki-code or HTML to the article, harming accessibility for the less technically inclined.
Substituting templates prevents newcomers from learning to use templates, and prevents users from finding their documentation.
If the template is just being used temporarily, it is usually better not to substitute. Substituted templates are much harder to remove or modify.
When a vandalised template is substituted, it is more difficult to repair than regular vandalism because of the lack of links between the template and its incarnations and the lack of updatability.
Some meta-templates, such as {{!}} and {{((}}, do not work if they are substituted.
Templates that should always be substituted
This is a list of templates that should always be substituted, organized by namespace. Each list is ordered alphabetically with grouped series. This is not a comprehensive list; other less-used templates may also need to be substituted, see the template's documentation for details.
Article namespace
{{No more links}}, substituting this inserts a commentary only visible in edit mode, with a short explanation to discourage spamming links.
This section is empty. You can help by adding to it.
Misc. templates
User signature templates, if transcluded, cause undue server strain as changes to a minor user template must then be widely propagated. If used, user signature templates should be substituted. (It should be noted, however, that user signature templates are discouraged.)
Uncategorized suggestions
This section is empty. You can help by adding to it.
{{nsd}}, for images with no source info, adds {{di-no source}} with today's date automatically filled in.
{{orfud}}, for "generic" orphaned fair use images, adds {{di-orphaned fair use}} with today's date automatically filled in.
{{or-fu-re}}, for fair use images that have been orphaned in favour of another image, adds {{di-orphaned fair use}} with today's date automatically filled in (takes the name of the replacement image as parameter).
{{rfu}}, for fair use images that it should be possible to replace with a free licensed image, adds {{replaceable fair use}}, with today's date automatically filled in.
{{refu-c}}, used in captions of images flagged as replaceable fair use, adds {{rfu-c}} with the "due date" (today + 7 days) automatically calculated and filled in.
{{OnProd}}, which inserts today's-date-dependent name of the category page to which the article was added when nominated for deletion.
contain external links as the formatting of the URL to perform queries or look-ups may change
This is because these constructs are not replaced in the generated wiki-code (single level substituting of case (A) leaves the #ifor#switch constructs verbatim at the subst location and (B) leaves constructs like {{{1|default value}}}).
If specific consensus is to eliminate a particular call of such a template, Special:ExpandTemplates can be used to expand that call to plain wiki-syntax.
List
This is a list of templates that should not be substituted. This is because they contain formatting standard code, complex code, or code that breaks if substituted.
Various citation/reference templates which contain very complex conditional code (see: WP:CITET).
{{Curlie}}, URL queries external web directories (former DMOZ).
Infobox templates, various ones for the sake of bot-identification and consistency.
{{Javadoc:EE}}, {{Javadoc:SE}}, {{Javadoc:SE-guide}}, conditional templates whose purpose is to update the links when the master template is updated.
{{ISP}}, {{Openproxy}}, {{repeat vandal}}, {{SharedIP}}, {{SharedIPEDU}}, {{s/wnote}}, not messages to the users (so it is no problem if they change from their original form), so no reason to subst, but the standard reasons not to subst apply.
{{tl}}, {{tlp}}, {{ttl}}, {{ti}}, {{tic}}, {{tls}}, {{cl}}, {{ccl}}, many very widely used, {{tl}} with over 7,090,000 transclusions at last count. Quicker to type than their substitutions, and non-substing them allows new Wikipedians to learn about them and be able to use them.
{{sockpuppet}} and similar templates intended to be placed on the top level page in user space. (Note: warnings and notices in the user talk space should be substituted.)
Note also that some things may appear to be templates but are actually magic words, such as {{!}} and {{noexternallanglinks}}. Substituting them either will output nothing, or will substitute a template that shadows them (like the deprecated Template:!).
Wrong title templates
These templates, used in the main-space, add a comment about why an article is misnamed. They may be removed as the MediaWiki titling facilities improve, and the wording/layout may change.
Most templates related to renaming or deleting pages are used temporarily, and thus do not need to be substituted (it just makes more work to delete them).
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