Google, Yahoo and Microsoft join hands in the drive to clean up the web of duplicate contents. The top three search engines are implementing support for the new HTML tag - “” .
The new tag defines the relationship between a document and an external resource that results to the site’s canonical or preferred URL form that would encompass all the sub-domains and variations of a website. The tag will tell search engine crawlers which canonical URL form it will use when retrieving search results.
The tag also putt the Canonical URL for at the forefront of the website content which will be used for accessing the page,* regardless of the session id, link parameter, sort parameter, parameter order of the URL form.
To implement Canonical URL form on website, site owners just need to add the following link tag at head section of the site’s HTML.
By adding the link tag, the search engine crawlers* will disregard the following URL variants but have the same contents as the identified Canonical URL form:
http://www.example.com/products?trackingid=feed http://www.example.com/products?sessionid=hgjkeor2 http://www.example.com/products?prin...ckingid=footer
The
Live Search Blog,
Google Webmaster Blog, and
Yahoo Search Blog identified some technical details to consider when implementing the tag on your sites.
Top 3 Search Engines Unite on Canonical URL 
