In Feb 2009 Google, Yahoo, and Microsoft announced support for a new link element to clean up duplicate urls on sites. The syntax is pretty simple: An ugly url such as http://www.example.com/page.aspx?cid=abc791283 can specify in the HEAD part of the document the following:
< link rel="canonical" href="http://example.com/page.html" />
That tells search engines that the preferred location of this url (the “canonical” location) is http://example.com/page.html instead of http://www.example.com/page.aspx?cid=abc791283 .