Early this year, the major search engines got together to announce a new tag that can be added to the <head> of a web document, commonly refered to as the canonical tag. This was a way of “fixing” duplicate content issues on a website, by ensuring any pages that were duplicated used the same canonical tag, to tell the search engines which version of a page that you want them to index.
This is particularly useful for shopping cart based websites where products in multiple categories can be indexed on multiple URLs, creating duplicate content issues as a result. By empoying a tag on each of these duplicated pages, you tell the search engines to only index one of these pages.
For example, with the Vertical Leap home page, if we had a canonical issue, it could be visible on www.vertical-leap.co.uk/ & www.vertical-leap.co.uk/index.html – and potentially both of these pages could be indexed. If for some reason we were unable to fix this “properly”, we could add the following tag to the home page to tell the search engines to only index 1 of these versions:
<link rel=”canonical” href=”http://www.vertical-leap.co.uk” />
However, Google in particular have announced that just adding this tag in does not automatically mean that they will follow this guidance.
These pages need to be substantially similar to one another, or Google will just ignore this tag. As Kerry announced a few months ago on this blog, Google have been working on cross domain support for the canonical tag and Google have now announced that the canonical tag is now supported across domains.
So for those webmasters that own multiple domains that are all pretty much the same, and for some reason have problems removing some of this content, or redirecting it appropriately, you can now use the canonical tag like you had been able to do previously on the 1 website.
It is still important to do like for like mapping using this tag – as mentioned above, the 2 (or more pages) utilising this canonical tag need to be substantially similar for this to be used by the search engines, so its important to only use this tag for this purpose.
This could be a really useful addition to the SEO toolset, though it does seem that this could be open to abuse by spammers – we’ll watch developments on this in the coming months, and assess whether or not this proves to be useful.
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