Google, Yahoo, and Microsoft announced a joint effort to help reduce duplicate content. The three major search engines came together to allow users to point out their preferred version of a url. This format offers users more control.
Duplicate content has been a challenging issue for a long time. Websites containing a lot of content such as a retail site, could end up with several urls for each page making it difficult for search engines to crawl.
Google gives the following example on the Webmaster Central blog:
<link rel=”canonical” href=”http://www.example.com/product.php?item=swedish-fish” />
Simply place this link tag in the head section of the duplicate content urls.
The tag can only be used on pages within a single site. Both absolute and relative links are acceptable, but the search engines recommend absolute links. Also, links to all urls will be directed to the one preferred url.
For more information, each of the search engines have explanations and examples in their own announcements: Google, Yahoo, and Microsoft.
