The rel=canonical tag (also referred to as simply the canonical tag) is an HTML element found in the head element of the page that helps prevent duplicate content issues.

A typical rel=canonical tag:

<link rel="canonical" href="" />

Oftentimes, a page's content can be accessed via multiple URLs, especially if using URL parameters, or a group of pages is so similar it's only worth crawling or indexing one of them. The rel=canonical element is a way for the webmaster to identify which of these URLs is representative of the group as the preferred page (also called the "canonical" page) and should be indexed.

For example, let's assume is a product listing page. is the same page with the sorting reversed. has the exact same content as the first page but has an additional URL parameter to indicate where the traffic was referred from. If all three of these URLs were crawled normally, they would be flagged as being duplicate content.

Instead, if we put a rel=canonical tag on all three of these pages that said is the canonical version of the page, search engines would know to index this URL and ignore the other two, thus eliminating the duplicate content issue.

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