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[1][2][3] as part of search engine optimization.
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 to http://www.example.com/page.html?parameter=1, even though both URLs return the same content.
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.[5] 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".[1]
While the canonical link element has its benefits, Matt Cutts, who is 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.[6]
canonical link element
<link rel="canonical" href="http://www.example.com/" />
<link rel="canonical" href="http://www.example.com/page.html" />
<link rel="canonical" href="http://www.example.com/directory/page.html" />