A few months ago search engines such as Yahoo and Google announced a new method of reducing internal duplicate content called the canonical tag. The canonical tag can be placed in the head section of any web page and is used to inform the search engine which page is the ‘main page’.

An example of the canonical tag would be:

<link rel="canonical" href="http://www.crazyaboutseo.co.uk/" />

Adding this tag onto any internal copies of my home page would inform Google of the main location of my home page, reducing internal duplicate content. The canonical tag works very similarly to a 301 redirect but does not redirect the canonical page to another URL.

301 redirects are still a much more efficient way of redirecting a duplicate page as a 301 redirect will pass any PR onto the new page. In addition the canonical tag can only work for pages on the same domain, and cannot be used as a method of reducing cross domain duplicate content issues, a 301 redirect will still be required here.

The canonical tag however is very useful and provides another tool in your arsenal to reduce duplicate content, without the hassle of complicated redirects

The canonical tag is great as it will inform the search engines as to which version to include in their index, this will help improve rankings for long tail keywords and improve the quality of their search index.

So in what cirumstances could you utilize this tag? Below I have detailed a few.

1. Product attributes

Many open source ecommerce stores create duplicate copies of their products. For example you may have a product available in a number of colours, with each colour being a new URL containing the same content as the original URL. You could change the system so that it uses JavaScript to change the colours, but this can be a long drawn out process. This is a great example of where to use the canonical tag. For example you could add the canonical tag to the ‘black colour’ product page informing Google of the location of the main product page

http://www.websitename.co.uk/product1?color=black
add the following tag
<link rel="canonical" href="http://www.websitename.co.uk/product1" />

2. Print version pages
Many website products have a print version which allows a user to view a print version of the page they are on. The issue here is that this again creates a duplicate page. To resolve this issue you could again use the canonical tag.

For example

http://www.websitename.co.uk/product1?print=true
add the following tag
<link rel="canonical" href="http://www.websitename.co.uk/product1" />

3. Session IDs indexed
Another common issue with ecommerce sites is that the search engines can index product and category pages with and without session IDs. The canonical tag can also resolve this problem, please see details below:

http://www.websitename.co.uk/trainers?SESSID=456
add the following tag
<link rel="canonical" href="http://www.websitename.co.uk/trainers" />

The examples shown above are only an indication of where the canonical tag could be used. You should contact your SEO company if you have any questions over the use of the tag. Although the search engines have publicly announced that they are now using this tag, they have stated that they currently may not adhere to the tag on every occassion.

My advice is if you have a large duplicate content issue to still use a 301 redirect. The canonical tag should only be used if:

• You have a small duplicate content issue
• You don’t have the knowledge to implement a 301 redirect
• You dont have access to the server to complete the redirect