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Using a template is similar to calling a function in programming. When called, it returns a value. For a template, calling is known as a template call and the return value is known as the expansion of the template, which is treated as part of the calling page's source. Like functions, some templates can take parameters that affect their output.

Calling a template causes it to be either transcluded or substituted in the page that calls it (the target page).

Transcluding a template means that when MediaWiki displays the page, it treats the template as if it were directly in the page’s source. Substituting a template differs in that it inserts the content of the template permanently into the target page when it is saved and subsequent editors cannot tell that it originated from a template.

When you update a template, every page that transcludes it automatically reflects the update when the page is displayed in the future, whereas updating a template has no effect on pages that have previously been saved with the template substituted.

To transclude a template, insert {{Template name}} at the point where it should appear in the page source. To substitute it, use {{subst:Template name}}. This wikitext is called a template call.

If the template only includes TemplateStyles (no visible content), although it could be called once anywhere on the page for all uses, it should be called near each usage. In the case of table styles, call it near and above each table that uses it. This avoids issues like missing when previewed/copied or flashing.

In MediaWiki, the wiki software that Wikipedia uses, variables are distinct from templates, though both are referenced by double curly braces {{ }} and they both return a value. These references differ in that variables in MediaWiki are always uppercase, while template names follow the same conventions and have the same limitations as page names. For example, they are case-sensitive (except for the first character), underscores are parsed as spaces, and template names cannot contain the characters # < > [ ] | { } because they are reserved for wiki markup.

The number sign # is used as a fragment identifier, which identifies a fragment or section of a page. Although you can use it in a link to link to a section of a template page (e.g., Template:Portal#Example), fragment identifiers have no meaning in a template call and are ignored. For example, {{Portal#Location|Books}} is the same as {{Portal|Books}}.

The template namespace is the default, so you can leave out the namespace Template: in the template name, and it is conventional to do so. However, you must specify the namespace prefix for templates in other namespaces, such as User:. To transclude a page located in mainspace, precede its page name with a colon, as {{:Page name}}.

Note: Attempting to transclude a template that does not exist produces a red link, just like linking to any other nonexistent page. Following the link allows one to create that particular template. It is not possible to transclude templates between projects (such as different-language Wikipedias or MediaWiki) – to use a template on another language project, a copy of the template must be created in that project.

The basic transclusion syntax for a template can be further controlled using parameters, which allow you to customize a template’s output. The syntax is {{Template name|parameter|parameter|...}}, where Template name is the template’s name, and each parameter can be either a simple value (known as an unnamed parameter) or in the form name=value (known as a named parameter). The first, second, third, etc., unnamed parameters are named 1, 2, 3, etc., so using {{Template name|1=value1|2=value2}} is equivalent to {{Template name|value1|value2}}.

Each template has specific parameters it can accept, as defined within its code. You can list named parameters in any order in a template call. Extra or misnamed parameters have no effect, while missing parameters cause a default value to be used. If a parameter is repeated, the last defined parameter value is used.

The value of a parameter can be an empty string, such as when the pipe (|) or equals sign is followed immediately by the next pipe or the closing braces. For example, {{Template name|1=|2=test}} produces the same output as {{Template name||2=test}}. This is different from not specifying the parameter at all, which results in a default value, although templates are often coded so as to behave the same in both cases.

If a template call specifies a parameter which is not defined in the template, it has no effect. Certain templates, especially complex ones like infoboxes, may use the check for unknown parameters module to alert editors about any unrecognized parameters they code by mistake. An editor who wants to add a comment relating to the template call can use the hidden text syntax: <!-- Comment -->

Note: If you wish to experiment with any of these, you can use the template sandbox, your userpage or sandbox. You can try out your template with Special:ExpandTemplates.

An example of a very simple template can be found at Template:TM, which expands to wikitext that places the trademark symbol (™), linked to the trademark article, at that point in the displayed page. A programmer would say that the template "returns" the trademark symbol link wikitext.

Display Template:TM (by clicking on the foregoing link), then click on the "Edit" tab to see the template code (its page source). The active part of that code, which becomes the expansion of the template when it is transcluded, is the single link wikitext [[Trademark|™]]. The remainder of the page source is enclosed between <noinclude> tags, so it shows up when you display the template page itself but not when you display a page that calls the template.

To transclude Template:TM onto another page (i.e., to use it on another page), type {{TM}} into the source of that page (the target page) and click Show preview. The page will be displayed with the template call replaced by the expansion of the template, as if the wikitext actually contained [[Trademark|™]] at that point. The displayed page will therefore contain the wikilink "™".

For example, type The trademark symbol is {{TM}} and you will see "The trademark symbol is ™" when previewing the page or after saving the change.

The other way to use a template is to substitute it. If you type The trademark symbol is {{subst:TM}} and preview or save the page, you will see "The trademark symbol is ™" just as with the transclusion example above. But if you save the page and then look again at the saved page source,^[b] you will see The trademark symbol is [[Trademark|™]], because the template call was replaced by the expansion of the template when you saved the page. There is no ongoing connection to the template call; no one can tell by looking at the page source that you didn't just type The trademark symbol is [[Trademark|™]] and if the {{TM}} template changes (or ceases to exist), your page will continue to display as it does now. (In contrast, in the transclusion example above, if the {{TM}} template subsequently changes, your page may then display differently).

{{payoff matrix | UL = 5 | UR = 7 | DL = 2 | DR = 9 | Name = Example usage }}

The following points may be worth noting when using templates:

• Templates are not the only method of transclusion, in some cases, other methods such as selective transclusion are more appropriate.
• An unnamed parameter (positional parameter) value cannot contain an ordinary equals sign, as this would be interpreted in the parameter specification as a named parameter (with the equals sign separating the name from its value).^[c]
  • To specify an unnamed parameter including an equals sign (for example in a URL with name–value pairs), replace the equals sign with the magic word {{=}}, which expands to an equals sign that will not be interpreted.
  • Another method is to explicitly specify the positional parameters. The first unnamed parameter is named "1" and so on. To call template {{done}} with a=b as the literal value for the first parameter, type either {{done|a{{=}}b}} or {{done|1=a=b}}.
• Similarly, it is not possible to use an ordinary pipe character | in a template parameter specification, as it would be interpreted as separating one parameter specification from another.^[d] This problem can similarly be solved by using the magic word {{!}} in place of the pipe, or—if the pipe is not intended to be parsed at a higher level—using the HTML entity |. Alternatively, for embedding wikitables in templates, you may use {{Wikitable}} to avoid excessive {{!}}.
• Remember that whitespace characters (spaces, tabs, carriage returns, and line feeds) are not automatically stripped from the start and end of unnamed parameters, unlike with named parameters. Including such characters (or any other non-visible characters in any parameters) may in some cases affect the template's behavior in unexpected ways. (Template designers can use {{Trim}} to remove unwanted whitespace in unnamed parameters).
• In documentation and discussions, it is customary to put the name of a template in double braces to emphasize the reference to a template (for example, use {{Trim}} as the name of Template:Trim). If you just type {{Trim}} in the source page, that will of course call the template, so to make it easy to display the name with the braces, and also make the name a link to the template for the reader's convenience, there is the {{tl}} template (the "template link" template). For example, {{tl|Example}} produces {{Example}}. There are various other template-linking templates available with other functions.
• When an editor changes a template (by editing the template or one of its subtemplates), the change will ultimately be reflected any time any page that transcludes that template is subsequently displayed. However, the change may not become visible on all pages immediately; a previously cached version of a page, based on the previous version of the template, may continue to be displayed for some time. Use the purge function to force a page to be displayed using the latest versions of templates—including on the template page itself, if it contains usage examples.
• When viewing old versions of pages, remember that templates will be transcluded as they are now, not necessarily as they were when the old page version was active.
• To list all pages that transclude a template, use the What links here link on the page when you display the template page. Note that this will not include pages where the template has been substituted, as there is no record of substitutions.
• To get a list of templates transcluded on a page, bring up the edit page and find the list below the edit window. This list also includes the subtemplates used by the templates that are directly transcluded. To get such a list for a page section, an old version of the page,^[e] or your newly edited version prior to saving, click Show preview on the appropriate edit page.
• There are limits to the number and complexity of the templates that an article may have. See the "§ Expand limits" section for help with this.
• If you want the template to leave a time stamp or signature, you can write <noinclude><nowiki></noinclude>~~~~~<noinclude></nowiki></noinclude>, but this works only when you substitute the template. If you transclude it, you'll just get ~~~~~.
• To improve readability, programmers usually like to split code with newlines and indent it. Unfortunately, MediaWiki does not allow this technique; in many instances, these purpose-built newlines are treated by MediaWiki as content. One possible workaround is to add <!-- before each newline character and --> after it, which produces an HTML comment.

Templates are just Wikipedia pages. They are created, deleted, and edited in much the same way as any other page.

Anything that can be included on a normal page or article can be included on a template, including other templates (called subtemplates). Templates often make use of programming features—parameters, parser functions, and other magic words—which allow the transcluded content to vary depending on context. There are also special tags to control which information is transcluded and which is not.

In template code, the value of a parameter is represented by items enclosed between triple braces, which is a parameter reference.

• The code {{{xxx}}} expands to the value of the parameter named "xxx".
• The codes {{{1}}}, {{{2}}}, and so on are expanded to the first, second, and so on unnamed parameters. (Note that an unnamed parameter can alternatively be specified in a template call as an equivalent named parameter named "1", "2", etc.).

If a parameter is not specified in the template call, then the parameter reference is not replaced with anything -- it is expanded literally; this means that if the template call does not specify the parameter "xxx", the wikitext {{{xxx}}} inside the template expands to literally {{{xxx}}} (not the null string you may have expected). You can get a more useful behavior by specifying a default value in the parameter reference. Do this with the pipe syntax: {{{xxx|dflt}}} specifies the default value dflt for the named parameter "xxx", and {{{1|dflt}}} specifies the default value dflt for the first unnamed parameter. Most often, one specifies a null default value, such as {{{1|}}} or {{{xxx|}}}.

If a call sets a parameter to the empty string like {{Template name|xxx=}} then {{{xxx|OK}}} will produce the empty string and not the default value "OK". Many users will expect that an empty parameter gives the same result as omitting the parameter. You can achieve this with: {{#if:{{{xxx|}}}|{{{xxx}}}|OK}}. This says: If xxx is assigned a non-empty value then use xxx, otherwise use "OK". If the default value is the empty string then you only have to write {{{xxx|}}}.

You can use default parameter values to effect a parameter alias: For example, if parameters "text" and "message" are names for the same parameter, which can also be specified as the only unnamed parameter, then refer to the parameter with {{{message|{{{text|{{{1|}}}}}}}}}. If the template call specifies more than one of those parameters, "message" will have priority, followed by "text", and finally by the first unnamed parameter. So if a template call specifies parameters |message=A|text=B|C, the above wikitext expands to A.

Because of the multiple meanings of double-brace and triple-brace syntax in wikitext, expressions can sometimes be ambiguous. It may be helpful or necessary to include spaces to resolve such ambiguity. For example, {{ {{{xxx}}} }} or {{{ {{xxx}} }}}, rather than typing five consecutive braces, may be more human-readable. But watch out for unwanted whitespace appearing in the template expansion.

Parameter references do not get expanded when they are wrapped in <nowiki> tags.

{| id="Payoff matrix" style="background:white; float: {{{Float|right}}}; clear:right; text-align:center;" align={{{Float|right}}} cellspacing=0 cellpadding=8 width={{{Width|225}}}
|-
|style="width:33%;                                                    "|
|style="width:33%;                     border-bottom: solid black 1px;"| {{{2L|Left}}}
|style="width:33%;                     border-bottom: solid black 1px;"| {{{2R|Right}}}
|-
|style="border-right: solid black 1px; text-align: right;             "| {{{1U|Up}}}
|style="border-right: solid black 1px; border-bottom: solid black 1px; background:{{{ULc|white}}}; font-size:120%; "| {{{UL|0, 0}}}
|style="border-right: solid black 1px; border-bottom: solid black 1px; background:{{{URc|white}}}; font-size:120%; "| {{{UR|0, 0}}}
|-
|style="border-right: solid black 1px; text-align: right;             "| {{{1D|Down}}}
|style="border-right: solid black 1px; border-bottom: solid black 1px; background:{{{DLc|white}}}; font-size:120%; "| {{{DL|0, 0}}}
|style="border-right: solid black 1px; border-bottom: solid black 1px; background:{{{DRc|white}}}; font-size:120%; "| {{{DR|0, 0}}}
|-
|style="font-size: 90%;" colspan=3 |''{{{Name|{{PAGENAME}}}}}''
|}

Template code often makes use of the variables and parser functions described at Help:Magic words to make the template's behavior depend on the environment in which it is included (such as the current time or namespace). Parser functions can be used for some arithmetic calculations and string manipulations on variables and parameter values, but certain standard programming features such as loops and variable assignment are not available. Full string manipulation is not available; some templates providing such function have been created, but they are inefficient and imperfect.

Some of the most frequently used variables and functions are listed below. For more, see Help:Magic words and the fuller documentation at the MediaWiki pages mw:Help:Magic words and mw:Help:Extension:ParserFunctions.

The ParserFunctions extension provides more programming-oriented parser functions:

The {{PAGENAME}} and {{NAMESPACE}}variables are particularly useful, and frequently used, to change template behavior based on the context in which they are included. Templates that contain category links often do this. For example, a cleanup template contains a category link to categorize the calling page as one which needs cleanup, so the template is likely to condition that category link on the {{NAMESPACE}} variable so that talk pages, user pages, and any other pages that might call the template incidentally do not get categorized as pages needing cleanup.

A template may call another template—this is called nesting and the called template is called, in this context, a subtemplate. When WikiMedia expands the template, it expands subtemplates as the calls to them appear, so that the final product is essentially the result of expanding templates from the most deeply nested out.

While fairly straightforward in application, it involves some noteworthy quirks and tricks.

To pass a parameter value from a template call to a subtemplate, use a parameter reference in the template call to the subtemplate.

Example:

Template:A contains "the quick brown {{B|{{{3}}} }} jumps over...". Template:B (a subtemplate) contains '''{{{1}}}'''. Page X calls A with {{A|apple|pear|fox}} This expands to "the quick brown '''fox''' jumps over...". The third unnamed parameter passed to Template:A gets passed as the first unnamed parameter to subtemplate B.

A template can even choose which subtemplate parameter to pass conditionally.

Examples:

Template:A contains the quick brown {{B|{{{3}}}=fox}} jumps over.... Template:B (a subtemplate) contains '''{{{jumper}}}'''. Page X calls A with {{A|apple|pear|jumper}}. This expands to "the quick brown '''fox''' jumps over...". The third unnamed parameter passed to Template:A is passed as the name of the parameter passed to subtemplate B with the value "fox".

Template recursion is not available; that is, a template may not call itself directly, or indirectly by calling other templates which call it. Attempts to do so will result in an error message describing a "template loop".

When a subtemplate contains unmatched braces—as in {{lb}}}—the unmatched braces are treated as text during processing—they do not affect the parsing of braces in the calling template. But where the template is substituted, the unmatched braces will be parsed as braces when the page is subsequently displayed. This has little practical use, but can occasionally introduce unexpected errors.

See mw:Manual:Advanced templates and mw:Manual:Recursive conversion of wikitext for more information.

By default, when a page calls a template, MediaWiki includes the expansion of the entire template in the calling page. However, it is possible to modify that behavior, using tags that specify which parts of the template code are to be included. This makes it possible for the template to contain information only for display when the template page is viewed, such as the template's documentation, or categories. It is also possible to have parts of the template be included when it's transcluded, but neither be not displayed when the template page itself is viewed nor be processed when the template page itself is saved (e.g., categories which do not apply to the template but need to be applied to pages which call or transclude the template). The tags are as follows:

• <noinclude>...</noinclude> – The text between the tags is not included when the template is called, but is processed when the template itself is viewed or saved; commonly used in template documentation.
• <onlyinclude>...</onlyinclude> – Nothing on the page except what appears between the tags is included when the template is called.
• <includeonly>...</includeonly> – The text between the tags is included when the template is called, but is not processed when the template is viewed or saved.

Perhaps the most common issue with the use of these tags is unwanted whitespace or linebreaks. It is important to remember that the effect of these tags begins immediately before the first opening angle bracket (<), not on the previous line or at the previous visible character; similarly the effect ends immediately after the last closing angle bracket (>), not on the next line or with the next visible character. For example:

}}<includeonly>

}}
</includeonly>

These tags can be nested inside each other, though (for a given page) this is significant only for the <onlyinclude> tag; nesting <includeonly> and <noinclude> tags is redundant. Be careful to properly nest the tags, however. Constructions like <onlyinclude>abc<includeonly>def</onlyinclude>ghi</includeonly> will not work as expected. Use the "first opened, last closed" rule that is standard for HTML/XML.

• The following techniques are helpful in debugging a template:
  • Use Special:ExpandTemplates to see the full recursive expansion of one or more templates.
  • Use subst: to substitute a template (rather than transclude it), which can show more clearly what is happening when the template is transcluded; see Help:Substitution.
  • Use msgnw: (short for "message, nowiki") to more-or-less transclude the source of the template rather than its expansion. It is not perfect: lists are rendered, comments are removed, and single newlines are replaced with spaces (which is particularly confounding when transcluding wikitext tables).
• If the first character of a template expansion (or parser function result) is one of four wiki markup characters—:, ;, *, #^[g], it is processed during display as though it were at the beginning of a line, even if the template call is not. This allows you to create various kinds of lists with templates where the template call may not be in the correct place for a list. To avoid this, use <nowiki /> before the markup or <nowiki>...</nowiki> around it, or use the HTML entities :, &#59;, *, #, or use the templates {{colon}}, {{;}}, {{asterisk}}, {{number sign}} (those templates may not exist at other wikis). In some cases, the HTML entities will work when the <nowiki /> does not. The problem often occurs when a parameter value in a template call starts with one of the four characters. See also {{Encodefirst}}.
• For issues with template substitution, such as how to control whether subtemplates are substituted as well when the parent template is substituted, see Help:Substitution.
• You can use the template {{Trim}} to strip any initial or final whitespace from unnamed parameter values if this would cause problems; named parameter values are automatically stripped in this way.
• To protect server resources and avoid infinite loops, the parser imposes certain limits on the depth of transclusion nesting and on the page size with expanded templates. This may cause a page to break if it uses very complex templates, particularly if there are multiple such templates on the same page. For more information, see WP:Template limits. You can check a page's overall load on the server by examining the generated HTML for a page and looking for the NewPP limit report comments.
• Do not use = wikimarkup to create a section header in a template which is intended for use in article space; this will create an edit link on a page that transcludes the template that will confusingly open the template for editing.
• You may avoid section edit links to the template by including <includeonly>__NOEDITSECTION__</includeonly>.

Documentation for users, together with the template's categories, normally goes after the template code, inside <noinclude>...</noinclude> tags. It is normally necessary to put the opening <noinclude> tag immediately after the end of the code, with no intervening spaces or newlines, to avoid transcluding unwanted whitespace.

In the case of complex templates, the documentation is often kept on a separate subpage of the template page (named "Template:XXX/doc"). This applies especially to many protected templates, so that non-administrators can edit the documentation. To do this, place a call to the {{Documentation}} template after the main template code and within <noinclude>...</noinclude> tags. If the "/doc" subpage does not exist, a link appears when you display the template that you can use to create the subpage.

The documentation subpage, rather than the template itself, is normally what is placed in categories to represent a template.

A redirect of a template functions as an alias. For example, Template:Tsh redirects to Template:Template shortcut, so you can code {{tsh|foo}} instead of {{Template shortcut|foo}}.

It is good to prepare template aliases for variations in whitespace and capitalization. For example, there is a template called {{See Wiktionary}}. The "W" is capitalized, since the word "Wiktionary" is so, but a redirect {{See wiktionary}} (with lowercase "w") exists because editors may misremember it as the latter.

It is common to use "template" to refer not only to a template, but to a template call, a template result, and a template name.

For example:

• I am going to put more detail in the {{short description}} template on this page.
• Upon seeing an {{under construction}} template, the reader may decide to come back later.
• There are many kinds of infoboxes. The templates all start with "Infobox".

These usages can be confusing, especially to someone less familiar with templates. Consider using more specific language.

• mw:Manual:$wgEnableScaryTranscluding
• mw:Manual:$wgNonincludableNamespaces
• mw:Manual:Parser.php
• mw:Manual:Parser functions

• WP:Database reports/Templates transcluded on the most pages—not counting transclusions through redirects
• Special:Allpages (namespace 10)—all pages in the Template namespace
• Special:ExpandTemplates
• Special:Unusedtemplates—templates which are not transcluded (but may be used for substitution)

• Help:User style—CSS and JS pages
• Help:System message—MediaWiki namespace pages
• m:Project portals—Multilingual portal pages
• WP:Book sources—used by Special:Booksources
• m:Spam blacklist
• Template:Example