Canonical URL Tag
In early 2009, Google, Yahoo!, and Bing announced they are all supporting the URL canonical tag. Traditionally, search engines would see URLs like rootbeer.php?coupon=AW as a different page than simply rootbeer.php, even though these pages were serving the same content. The parameters being sent were used solely in the backend. While this made no difference to visitors to the page, it caused a duplicate content problem for webmasters.
This newly supported tag allows webmasters to tell the search engines that a duplicate page is occuring and specifies which is the "true" page.
Google has mentioned that the URL Canonical Tag is handled the same way as 301 redirects, making this a very SEO friendly method to convserve your PageRank and avoid duplicate content.
<link rel="canonical" href="http://www.domain.com/page.php" />
This code must be placed between the <head></head> tags.
When To Use It
If you are using an SSL certificate on your website, try using the URL canonical tag on pages which are OK to be accessed via HTTP.
If your website has results or product/article/item indexing which spans multiple pages using a "page" parameter, consider using the canonical tag when the parameter is set.
If you are sending parameters to pages which are not effecting the display on the front-end, implementing the URL canonical tag may be a good idea.
If yourdomain.com and yourdomain.com/index.php display the same thing, try adding the URL canonical tag to display when the index or default filename is in the URI
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