Google have just announced that it now supports the canonical tag across all domains which is great news if you have lots of duplicate content and you're worried about whether or not it'll have a negative impact on your site's performance.
If Google sees that you have the same content on a couple of pages, it may only index one of those pages. But what happens if it indexes the wrong page? Now Google supports a canonical tag so you can tell Google which page is more useful to you and tell it which one you want to index. Sometimes it's not possible to do a redirect so this is a useful tag to know about.
So, the big question, how do you set it up?
Here are the instructions from Google Webmaster Central and there's a nice video from Matt Cutts as well.
"How do I specify a canonical page?
To specify a canonical link to the page http://www.example.com/product.php?item=swedish-fish, create a <link> element as follows:
<link rel="canonical" href="http://www.example.com/product.php?item=swedish-fish"/>Copy this link into the <head> section of all non-canonical versions of the page, such as http://www.example.com/product.php?item=swedish-fish&sort=price.
If you publish content on both http://www.example.com/product.php?item=swedish-fish and https://www.example.com/product.php?item=swedish-fish, you can specify the canonical version of the page. Create the <link> element:
<link rel="canonical" href="http://www.example.com/product.php?item=swedish-fish"/>Add this link to the <head> section of https://www.example.comproduct.php?item=swedish-fish."
More information about using this tag is available at "About rel=canonical" at Google Webmaster Central or take a look at the video below.
