In this article I will try to explain you about Google’s brain child html tag called “Canonical Link tag”. If you are a owner of a website or an SEO or a webmaster you must be facing problems with duplicate content, canonical pages etc; The scope for this problem will be bigger if your website promotes identical, similar content which can be accessible through multiple links (URLs). The same content of your website can be displayed through different URLs. For example,

mywebsite.com can be

• mywebsite.com
• www.mywebsite.com/
• mywebsite.com/
• www.mywebsite.com/index.html
• mywebsite.com/index.html
• www.mywebsite.com/Home.aspx
• mywebsite.com/Home.aspx

By looking at each of the above URLs we think they are one and the same. But they are technically different. Though they all lead to the same content in most of the cases it is a big problem when a search engine has to retrieve the exact page. This problem is referred as “canonical pages”. This problem can be solved by following few methods specified below:

1. Normalizing: Using the Content Management System (CMS) to generate only the urls you want to show up.
2. Picking one “canonical” url and ensure you link consistently within your web site
3. By making all non canonical urls do a permanent(301) HTTP redirect to the canonical/preferred url
4. Break ties in google by submitting your preferred url in a sitemaps file
5. Using Google’s web master tools and specifying like www type or non – www type.

But there are few problems which will not be solved by actions mentioned in the above solutions. Few times generating permanent links/ 301 redirects will be a hassle and cannot be done. Another problem is the linking, you cannot control how people/users link to the URL. Also, the case sensitive issues, session id problems. The tracking codes, landing pages and analytics are major problems too.

So Google has come up with a proper solution for this, A format in which you can be able to specify what is your preferred version of a URL. They came up with a new tag called “canonical link tag”, which allows you to do the same.

Yahoo’s response to this is “We recommend that you structure your site with normalized URLs and minimum duplication, or use 301s if need be. If those don’t work for you, try Site Explorer and/or the tag. Our support for the tag will be implemented over the coming months.”

And Microsoft’s opinion on it is like this: ‘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 through domain canonicalization and normalization of URL parameters. We’ll provide more details on the link tag after we’ve implemented full support in one of our upcoming releases. In the meantime, we look forward to hearing your feedback on the new tag.”

Ask.com search engine is also supporting this feature.

Hence this canonical link tag will solve the so many problems (which occur very commonly)like, improper linking to a page which also displays the same content , case sensitive issues and forward slash problems.

Whenever redirecting to the above “preferred” URL the URL properties will also be copied.

Here we need to keep few things in mind, when using the canonical tag the pages on which you are using this tag must contain identical content. They need not be exactly same and minor differences are acceptable but they should be almost similar. Google follows chain of rel = “canonical” designations, but they suggest to update the links to a single canonical page to ensure the optimal canonicalization.

The canonical tag is not a directive but a hint the search engines will respect strongly. It is a suggestion which the search engine will keep in mind while calculating the search results. So, when the search engine calculates the accurate URL according to its algorithm it will take your preference into account and displays the link.

You can add relative paths like in the canonical link tag like:

And the rel=”canonical” can be a redirect too. In case the canonical specified URL does not exist, there will not be a problem as the search engine first indexes the pages and then only goes through the canonical tag. And the search engine will also support the cross domain canonical link element i.e, the canonical link tag specified may refer to a canonical URL on a completely different domain.

Hence I conclude that, the canonical link tag is a very good if not the best way to get rid of the duplication problems in website URLs.