I’ve seen reference to canonical tags before with respect to SEO (search engine optimization). What is it, and must I use them in the design of my website in order to be indexed by Google and other search engines?
Michael Cyger
[ Admin ]
On February 12, 2009, Google introduced the canonical tag (http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html).
The problem that many websites faced in the past (and many still face today) was that they used a content management system and displayed webpages using multiple URLs. For example, using the Wordpress content management system you might be able to access a page about your cats at http://www.mysite.com/my-cats or as http://www.mysite.com/?p=15. Both of these URLs might redirect (http://www.page2sucks.com/questions/seo-301-or-302-redirects) to one or the other URL, or they might stay as those distinct URLs. In both cases, they would show the exact same content.
Now, duplicate content in search engines is bad (look at the extreme…what if Google showed 100 URLs of results – the first 10 pages of results – that were all the exact same content page but on different URLS?). So in the past Google has tried to figure out which of the two URLs you prefer (http://www.mysite.com/my-cats or http://www.mysite.com/?p=15) and would put your PageRank into that URL. In the worst case scenario, Google would think you're trying to "game" their system by submitting multiple pages with the same content just to rank multiple pages highly, and would exclude or penalize some of the pages.
Today, Google and other search engines are more sophisticated. They realize that you may not be able to control every instance of a URL using third party systems. Instead, they've come up with the canonical tag.
<link rel="canonical" href="http://www.mysite.com/my-cats/" />
This simple tag, included within your webpage, will tell Google, Bing! and other search engines that this URL (http://www.mysite.com/my-cats/) is the preferred URL to be indexing and storing in their search database. And if they come across any other URL showing this exact same content, please index it and put it towards this URL (http://www.mysite.com/my-cats/).
For more information on the canonical URL, please see Google's Webmaster Central Blog entry at http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html.
According to the Google blog, This link-tag is currently also supported by Ask.com, Microsoft Live Search and Yahoo!.
Here is Matt Cutts discussing the canonical link element via video: