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Sometimes it is undesirable to allow access to a template via a URL. For instance, a template that you only use as an embedded template would likely be an incomplete HTML document, and not something you would want your users to be able to come across on its own, even accidentally.
"Hidden" templates are just that: templates that cannot be accessed from a URL, but can be used as embedded templates. What makes a template "hidden"?
ExpressionEngine uses a convention that is common among many file-systems, so it may already be familiar to you. To make a template "hidden", simply give it a name preceded by a period, e.g. ".my_hidden_template".
When a Hidden Template is Requested in the URL
When someone attempts to access a hidden template via the URL, one of two things will occur. If you have specified a 404 template in your Global Template Preferences, then the 404 template will be displayed, with 404 headers. If you have not specified a 404 template, then the index template of the requested template group will be displayed.
In this example, if the URL http://example.com/index.php/site/.navigation was requested, the site's 404 template would be shown if it was defined, or the site/index template if it were not.