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Including the contents of a template page on a different page (by surrounding the page name with double braces, i.e. "{{}}" and leaving out the "Template:" prefix) is called transclusion.
This allows the other page to be automatically updated whenever the template page is updated.
Besides templates, pages in the following namespaces can be transcluded:
The only difference between transcluding these and templates is that you must include the prefix (followed by a colon) with the page's name inside the double braces.
Images also can be transcluded, but this is done using double square brackets instead of braces.
Here are some examples of transclusion:
{{Pic of the day}} · {{Help:Contents}} · [[Image:Wikipedia-logo.png]]
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