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In Wikipedia, transclusion is the inclusion of content from a source page, often a template, into the content of a target page. This happens each time the target page is viewed. Transclusion differs from substitution, which copies the source page into the target page once and permanently.
Similarly to creating a wikilink using double square brackets ([[Pagename]]), a page can be transcluded as a template by enclosing its title in double curly braces or double curly brackets: {{Namespace:Pagename}}. Any changes made to the source page, or template, are automatically reflected on all pages that include the transcluded content.