Canonical link element

A canonical link element is an HTML element that helps webmasters prevent duplicate content issues in search engine optimization by specifying the "canonical" or "preferred" version of a web page. It is described in RFC 6596, which went live in April 2012.[1][2]

Purpose

A major problem for search engines is to determine the original source for documents that are available on multiple URLs. Content duplication can happen in many ways, including:[3]

  • Duplication due to GET-parameters
  • Duplication with multiple URLs due to CMS
  • Duplication due to accessibility on different hosts/protocols
  • Duplication due to print versions of websites

Duplicate content issues occur when the same content is accessible from multiple URLs.[4] For example, http://www.example.com/page.html would be considered by search engines to be an entirely different page from http://www.example.com/page.html?parameter=1, even though both URLs may reference the same content.[5][6]

In February 2009, Google, Yahoo and Microsoft announced support for the canonical link element, which can be inserted into the <head> section of a web page, to allow webmasters to prevent these issues.[7] The canonical link element helps webmasters make clear to the search engines which page should be credited as the original.

How search engines handle rel=canonical

Search engines try to utilize canonical link definitions as an output filter for their search results. If multiple URLs contain the same content in the result set, the canonical link URL definitions will likely be incorporated to determine the original source of the content. ″For example, when Google finds identical content instances, it decides to show one of them. Its choice of the resource to display in the search results will depend upon the search query.″[8]

According to Google, the canonical link element is not considered to be a directive, but rather a hint that the ranking algorithm will "honor strongly."[1][9]

While the canonical link element has its benefits, Matt Cutts, then the head of Google's webspam team, has said that the search engine prefers the use of 301 redirects. Cutts said the preference for redirects is because Google's spiders can choose to ignore a canonical link element if they deem it more beneficial to do so.[10]

Implementation

The canonical link element can be either used in the HTML <head /> or sent with the HTTP header of a document. For non HTML documents, the HTTP header is an alternate way to set a canonical URL.[3][11]

By the HTML 5 standard, the <link rel="canonical" href="http://example.com/"> HTML element must be within the <head /> section of the document.[12]

Examples

HTML

Below you can see an example of HTML code that utilises the rel=canonical inside the <head> tags. The code could be used on a page such as https://example.com/page.php?parameter=1to tell search engines that the https://example.com/page.php is the preferred version of the webpage.

<!DOCTYPE html>
<html>
<head>
  <link rel="canonical" href="https://example.com/page.php" />
</head>
<body>
...
</body>
</html>

HTTP

 
HTTP/1.1 200 OK
Content-Type: application/pdf
Link: <https://example.com/page.php>; rel="canonical"
Content-Length: 4223
...
gollark: Hmmmm. I have no idea how to run this.
gollark: Prove it.
gollark: Also, what?
gollark: Which Microsoft?
gollark: One which would TOTALLY NOT install potatOS on all computers.

See also
  • URL normalization

References
  1. Kupke, Joachim (2009-02-12). "Specify your canonical". Google. Retrieved 2012-08-02.
  2. Cutts, Matt (2009-02-15). "Learn about the Canonical Link Element in 5 minutes". Retrieved 2012-08-02.
  3. "Link rel=canonical: How to do URL canonicalization right". Audisto GmbH. Retrieved 2015-10-06.
  4. "Duplicate content". Google. Retrieved 2012-08-02.
  5. Biswas, Kushal. "Canonical Issue and How to Use Canonical Tag – The Proper Way". RevenueI. Archived from the original on 14 June 2016. Retrieved 18 June 2015.
  6. Zadro, Dario. "Rel=Canonical - A Beginners Guide to Canonical Tags - Where and When to Use Them". Zadro Web. Retrieved 18 June 2015.
  7. Fox, Vanessa (2009-02-12). "Google, Yahoo & Microsoft Unite On "Canonical Tag" To Reduce Duplicate Content Clutter". Search Engine Land. Retrieved 2012-08-02.
  8. How Google And Other Search Engines Manage Canonical Links | http://seomediax.com/seo/how-google-and-other-search-engines-manage-canonical-links/
  9. "Consolidate duplicate URLs - Search Console Help". support.google.com.
  10. Cutts, Matt (2011-05-16). "A rel [equals] canonical corner case". Retrieved 2012-08-02.
  11. "Consolidate duplicate URLs - Search Console Help". support.google.com.
  12. "HTML link tag". www.w3schools.com. Retrieved 2019-01-07.
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