Canonical link element
A canonical link, English Canonical Link, website operators are in an HTML document, the opportunity in content repeatedly used ( duplicate content ) disclose the original resource. He is described in RFC 6596 and is listed in the form of a link element in the head of an HTML document:
If one uses XHTML instead of HTML, so a trailing slash is required.
It often happens that multiple URIs refer to the same content. Examples are dynamically generated content, such as. In forums or other content management systems, where the same items at different addresses are available, print views of web pages or local copies of files, such as licenses In these cases can be specified using the Canonical Link the primary URI where the content should be accessible. Without this information there is a risk that search engines index the contents of arbitrary and may consider the duplicate as a primary resource.
Linking party websites, especially search engines with their link databases, being able to find the original document to one side and refer to it in the search results. The attribute of search engine operators - especially Google - introduced to make it easier for their crawlers to reliably detect the primary resource.