The search engines Google, Yahoo and MSN have united to help webmasters resolve any duplicate content issues they may have within their own domains. This will be a huge help to many web site owners once they get their head around how to use it, and once the search engines their get their heads around how to use it as well.

Duplicate content can occur for many reasons. A simple example is with a blog where you have a page for each post with the title in the url. If a category search is done then the page may well appear with the category in the url, likewise for archives where the month may appear in the url. You now have three pages with slightly different url’s but exactly the same content.

You can now specify the single url that all duplicate pages should reference. This is done through a dedicated tag in the header section of your pages and should be written like:

link rel=”canonical” href=”http://www.YourPreferredURLHere” /

As with all tags they should appear between . The canonical fix can also be used to clear any www – non www issues within your domain as well.

For the search engines, this tag will not be considered an instruction – it will be considered an advisory. However they all agree that it will be used initially as a very strong advisory. To quote Google’s stance:

Is rel=”canonical” a hint or a directive?
It’s a hint that we honor strongly. We’ll take your preference into account, in conjunction with other signals, when calculating the most relevant page to display in search results.

There are a few important points to consider when using this tag:

It cannot be used to transfer from one domain to another – it can only be used within a domain
The tag will act as a 301 and transfer everything including links and page rank to the preferred URL
Tags can be absolute or relative although the recommendation is to use absolute paths
Support for the tag is not instant – it may take several months for full implementation (Yahoo and MSN)

I think MSN sums up the general attitude to this tag:

While we expect this command will help us solve many of the more complex duplicate content issues, we still highly recommend that webmasters follow the existing best practices for normalizing their URLs

Do everything possible to reduce duplicate content issues by working on your URL’s. With some blog platforms you can ‘nofollow’ duplicate URL’s such as those found in category or archived listings. The trap to avoid is to consider this tag as a panacea and not to bother with any internal planning. If this happens en-mass I can see the search engines dumping it citing abuse.

Used with thought it can help to reduce any remaining duplicate content issues within your web site. It should be noted that for search engines like Google, duplicate content within your own site is not a big issue. Google already makes an assumption as to which URL is the most important and effectively 301’s everything to it anyway.

It is strange to see the search engines working together on an issue and coming out with simultaneous posts on the issue. When it comes to SEO, it is probably the only thing the search engines agree on at present.

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This entry was posted on Sunday, February 15th, 2009 at 3:18 am and is filed under Search Engines. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.