Canonical Tag Announced: Google’s Matt Cutts Interviewed
Eric Schmidt once famously (or infamously, depending on how you look at it) called the Internet a cesspool. Now Google along with search rivals Yahoo and Microsoft are working together to clean up that cesspool to some extent.
Much of this "cesspool" comes from duplicate content, and a tag has now been revealed jointly from the three search engine giants that can give your pages the URL format that they all prefer. This is called the Canonical Tag and looks like this:
<link rel=”canonical” href=”http://www.example.com/product.php?item=swedish-fish” />
As Google explains (see here for example, and FAQ), you can simply add this tag to specify your preferred version of a URL inside the <head> section of the duplicate content URLs:
and the search engines will understand that the duplicates all refer to the canonical URL: http://www.example.com/product.php?item=swedish-fish. Additional URL properties, like PageRank and related signals, are transferred as well.