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On-Site Duplicate Content No More: Canonical URL Tag

posted on February 13, 2009


Think your site is affected by a duplicate content penalty? The big three have good news for you: Google, Yahoo and MSN announced a new tag this week to specify the canonical URL for each page on your site.

Duplicate content has long been an issue for search engines and marketers alike. Search engines want to avoid cluttering results with copies of the same text while marketers and website owners often need (or create, whether or not they need them) several copies of the same text with different URLs on their sites for usability or other purposes.

The new tag allows webmasters to specify what URL the search engines should use for the content, regardless of what session id, link parameter, sort parameter, parameter order or other variable has been appended to the end of the URL in a link. The new tag goes in the <head> section of a page:

<link rel="canonical" href="http://www.example.com/product.php?item=swedish-fish" />

Google’s and Yahoo’s blog entries address some common questions as well. rel="canonical&quot (the Canonical Link Tag) is a “a hint that we honor strongly” according to Google, who vouches to “take your preference into account, in conjunction with other signals, when calculating the most relevant page to display in search results.”

The Canonical Link Tag can work with relative URLs and <base> links, but both teams recommend using absolute URLs in the Canonical Link Tag. Different subdomains for a canonical URL vs a display URL are okay; however, the tag cannot be used to specify a URL on a different domain as the canonical URL of a page. Instead, they recommend the standby 301 redirect solution

The URL specified in the rel=”canonical” tag can be a redirect to a canonical URL, but they don’t recommend specifying a 404 page as a canonical URL. The tag allows slight differences in content (Google gives the example of sort order on a table of products).

As a “strong hint,” the big three don’t promise to always use the canonical URL. In fact, Google says:

Our algorithm is lenient: We can follow canonical chains, but we strongly recommend that you update links to point to a single canonical page to ensure optimal canonicalization results.

So despite this effort, a lot of links to a noncanonical URL can still influence what URL the search engines use—or can undermine your link equity.

Google cites a live example at http://starwars.wikia.com/wiki/Nelvana_Limited, which specifies its rel=”canonical” as http://starwars.wikia.com/wiki/Nelvana. Wikipedia has long had a problem with noncanonical URLs, so seeing Wikia’s participation is a sign that they’ll work to resolve this issue on the encyclopedia, too.

Duplicate content has been a major concern for years. Will you be implementing this tag on your sites?


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  • http://www.seofox.com David Ogletree

    This tag is going to be nice to have but I still think you should enforce URL’s. You should know every possible URL that can be access on your site. It should be impossible to bring up the same content with more than one URL. There should be a system in place that 301 redirects any attempt to access content with the wrong URL. Matt Cutts told me one day that it is a good idea to also enforce capitals. The best way is to just make sure all url’s are lower case.

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  • http://www.adaid.com.au/internet_marketing.aspx Internet Marketing

    So, what if the site is not dynamic? That may be quite a costly exercise to put all of that into code.

  • http://www.seofox.com David Ogletree

    If your site is static then you don’t need this tag. It is only for static sites. If you have dup problems on your static site just delete the duplicate pages.

  • http://www.freedownloadsok.com Freedownloadsok.com

    I just hear this tag from this blog. I think i have to learn more about this tag..

  • http://www.velvetblues.com Shirley – Velvet Blues

    Well this is useful for us bloggers who use excerpts or full posts on our index or archive pages.

    Shirley – Velvet Blues’s last blog post..Is Your Search Engine Optimization Expert Legit?

  • http://www.exceedonline.co.nz Kyle Aspinall

    What if you have two blogs with the same article? We are looking to replicate our website for another country, make tweaks to case studies and services based on our service offering for that country, but were looking to use duplicate blog articles…. not all the time but sometimes. How would this help? Or is it best to not even consider it?

  • http://aboutnewtech.com Dave Williams

    I am using this as of today with the new canonical name (SEO NO DUPLICATE) wordpress plugin. You can check it out at the wordpress.org plugin page.

    It seems to work. It also seems way to easy a solution.

    The fact remains that it is completely valid to index your content in multiple ways depending on the visitor or the site section they are in. Multiply indexes and you multiply content. It’s a sticky wicket.

    Thanks for this post. It helped me.

    Dave Williams’s last blog post..Microsoft Windows 7 Beta – Game, Application and Utility Compatibility – What’s Stable & Works, What’s Doesn’t Work and What To Do About It!

  • http://centurycafe.com NYC Cafe

    This could be a little help with keyword mapping and actually pagerank sculpting on pages with a lot of similar content.

    NYC Cafe’s last blog post..Want Flowers on a Cake – Check out this !!!

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