Canonical link element
A canonical link element is an HTML element that helps webmasters prevent duplicate content issues by specifying the "canonical", or "preferred", version of a web page as part of search engine optimization. It is described in RFC 6596, which went live in April 2012.
Duplicate content issues occur when the same content is accessible from multiple URLs. 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 return the same content. Another example is essentially the same (tabular) content, but sorted differently.
Canonical tags can also be useful to solve www and non-www duplicate content—where two URLs, identical except that one begins with "www" and other does not, point to the same page. This particular problem can be solved by proper use of the canonical tag.
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. The canonical link element helps webmasters make clear to the search engines which page should be credited as the original.
According to Google, the canonical link element is not considered to be a directive, but a hint that the web crawler will "honor strongly".
While the canonical link element has its benefits, Matt Cutts, the head of Google's webspam team, has claimed that the search engine prefers the use of 301 redirects. Cutts claims the preference for redirects is because Google's spiders can choose to ignore a canonical link element if they feel it is more beneficial to do so.
<link rel="canonical" href="http://example.com/" />
<link rel="canonical" href="http://example.com/index.php" />
- Kupke, Joachim (2009-02-12). "Specify your canonical". Google. Retrieved 2012-08-02.
- Cutts, Matt (2009-02-15). "Learn about the Canonical Link Element in 5 minutes". Retrieved 2012-08-02.
- "Duplicate content". Google. Retrieved 2012-08-02.
- Fox, Vanessa (2009-02-12). "Google, Yahoo & Microsoft Unite On "Canonical Tag" To Reduce Duplicate Content Clutter". Search Engine Land. Retrieved 2012-08-02.
- Cutts, Matt (2011-05-16). "A rel [equals] canonical corner case". Retrieved 2012-08-02.