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{{A note}} Attempting to transclude a template that does not exist produces a [[WP:Red link|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.
=== Parameters ===
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{{Further|Help:Transclusion#Template parameters}}
{{shortcut|H:PARAMETER|WP:PARAMETER}}
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 <code><nowiki>{{Template name|</nowiki>''parameter''<nowiki>|</nowiki>''parameter''<nowiki>|...}}</nowiki></code>, where <code>{{var|Template name}}</code> is the template’s name, and each <code>{{var|parameter}}</code> can be either a simple value (known as an ''{{dfn|unnamed parameter}}'') or in the form <code>{{var|name}}={{var|value}}</code> (known as a ''{{dfn|named parameter}}''). The first, second, third, etc., unnamed parameters are named <code>1</code>, <code>2</code>, <code>3</code>, etc., so using <code><nowiki>{{Template name|1=value1|2=value2}}</nowiki></code> is equivalent to <code><nowiki>{{Template name|value1|value2}}</nowiki></code>.
[[Parameter|Parameters]] allow customization of a template’s output. The syntax is <code><nowiki><{{Template name|parameter|parameter|...}}</nowiki></code>, where <nowiki><code>Template name</code></nowiki> is the template’s name. Each <nowiki><code>parameter</code></nowiki> can be a simple value (an ''unnamed parameter'') or a <nowiki><code>name=value</code></nowiki> pair (a ''named parameter''). Unnamed parameters are referred to by numbers like 1, 2, 3, etc., so <nowiki><code>{{Template name|1=value1|2=value2}}</nowiki><nowiki></code></nowiki> is equivalent to <nowiki><code>{{Template name|value1|value2}}</nowiki><nowiki></code></nowiki>.
Whitespace characters (spaces, tabs, returns) are stripped from the beginnings and ends of {{em|named}} parameter names and values, but not from the middle: thus <code><nowiki>{{ ... | myparam = this is a test }}</nowiki></code> has the same effect as <code><nowiki>{{ ... |myparam=this is a test}}</nowiki></code>. This does not apply to {{em|unnamed}} parameters, where all whitespace characters are preserved.▼
==== Whitespace handling ====
▲
The value of a parameter can be the empty string, such as when the pipe or equals sign is followed immediately by the next pipe or the closing braces. 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.▼
<!-- Leading and trailing [[Wikipedia:Whitespace|whitespace]] around {{em|named}} parameters and values is trimmed and removed automatically, but spaces within the maddle of the value itself are kept. For instance, <code><nowiki>{{ ... | myparam = this is a test }}</nowiki></code> has the same output as <code><nowiki>{{ ... |myparam=this is a test}}</nowiki></code>. However, with {{em|unnamed}} parameters, all whitespace are retained as preserved in the output. -->
If a template call specifies a parameter which is not defined in the template, it has no effect. Sometimes editors do this on purpose: For example, {{para|reason}} is frequently used as a pseudo-parameter to explain briefly in the wiki source why the template call was put there.{{efn|Some templates, such as {{tlx|Requested move}}, have code to display {{para|reason}} as visible output; whether to do so is determined on a template-by-template basis.}} Some templates use {{mlx|check for unknown parameters|check|...}} to warn the editor if a parameter is being used that is not accounted for in the template's code; this is mostly used for [[Help:Infobox|infoboxes]] and other templates with a large number of complicated parameters, where the presence of an unknown one is usually an unintentional error. If you update such a template to define a new parameter, its call to the module must also be updated to include the new parameter.▼
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 are ignored, while missing parameters will use a default value. If a parameter is repeated, the last defined parameter value will be used.
▲The value of a parameter can be
Unused parameters have no impact. Sometimes, editors include a parameter like {{tag|reason|open}} to add a brief explanation within the source as a [[Wikipedia:Manual of Style/Hidden text|hidden comment.]] However, some templates, like [[Template:Requested move|Requested move]], are programmed to show the reason parameter if provided. Certain templates, especially complex ones like [[Wikipedia:Infobox|infoboxes]], may use the check for [[Module:Check for unknown parameters|unknown parameters module]] to alert editors about any [[:Category:Unknown parameters|unrecognized parameters]] to help avoid unintentional errors.
==== Handling variable-length variables ====
In MediaWiki, templates can’t automatically handle an unknown or unlimited number of parameters. Each possible parameter usually has to be predefined in the template code. For example, a template might be set up to use three specific parameters, such as <code>1</code>, <code>2</code>, and <code>3</code>. If someone includes additional parameters beyond those, they wil not be recognized or processed by the template.
However, there are some ways to work around this:
1. Setting a limit: You can write the template to handle a fixed number of parameters by manually specifying each one (e.g., up to 10 or 20 parameters).
2. Using templates or [[Wikipedia:Lua|modules]]: For cases needing a flexible number of inputs, templates can use Lua modules or helper templates (like <nowiki>{{separated entries}}</nowiki>). These helper tools provide more advanced handling, such as counting or iterating through parameters, enabling the use of multiple inputs without setting an exact number.
However, modules can bypass this limitation. For simpler cases, the separated entries module expands all sequential parameters and lets you set custom delimiters. For more advanced usage, the params module enables counting, listing, mapping, filtering, and handling a variable number of parameters without prior knowledge of the exact number.
This workaround allows templates to ''appear'' as if they handle a variable number of parameters by processing each one iteratively up to a certain point, but true infinite flexibility is not built into the core MediaWiki system.
If a parameter is intentionally left blank (e.g., {{para|1}|open}}}), it is treated as an empty string, which differs from leaving it out altogether (in which case the default value applies). However, templates are often coded to act the same in both cases.
Sometimes, editors may add parameters that aren't. not recognized by the template. This can be useful, for instance, when using reason to add a brief explanation directly in the source. Certain templates use [[:Category:Error-handling modules|error-checking modules]] like check for unknown parameters to notify the editor of any unrecognized parameters, mainly in complex templates like infoboxes, where such errors are common.
▲If a template call specifies a parameter which is not defined in the template, it has no effect. Sometimes editors do this on purpose: For example, {{para|reason}} is frequently used as a pseudo-parameter to explain briefly in the wiki source why the template call was put there (a form of a [[Wikipedia:Manual of Style/Hidden text|hidden comment]]).{{efn|Some templates, such as {{tlx|Requested move}}, have code to display {{para|reason}} as visible output; whether to do so is determined on a template-by-template basis.}} Some templates use {{mlx|check for unknown parameters|check|...}} to warn the editor if a parameter is being used that is not accounted for in the template's code; this is mostly used for [[Help:Infobox|infoboxes]] and other templates with a large number of complicated parameters, where the presence of an unknown one is usually an unintentional error. If you update such a template to define a new parameter, its call to the [[Wikipedia:Module|module]] must also be updated to include the new parameter.
Wikitext syntax has limits and does not natively support templates with an unlimited [[variadic template|variadic parameters]]. However, this restriction can be circumvented using dedicated Lua modules. For simpler cases, separated entries can handle {{mlx|separated entries|main}} and add custom delimiters. For more advanced use, {{mlx|[params}} can dynamically count, list, or map all provided parameters without needing to know their count in advance.
===Examples===
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