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Rel canonical is a tag in the header of your HMTL that tells search engines which piece of content is the original and which one is a duplicate. It has been created in order to avoid duplicate content issues and optimize your SEO.
Indeed, some pieces of content can appear in multiple places of your website and thus be seen as duplicate content. As we know, duplicate content is penalized and rel canonical provides search engines a way to know which version of the content is the original, gives credit to that primary one, links the copy to the right URL and thus displays the good version in the search engines results.
Canonicalization is essential to create a well optimized website and offers a better user experience; users do not have to choose which version of a page is the best or more likely to be the original.
There are multiple situations where duplicate content is relevant and should not be penalized:
Actually, these examples are not true duplicate contents, they are system generated URLs. It means there are different URLs serving the same content but rel canonical should be used to tell search engines which one is the original content, and which URL should be crawled, indexed and returned on SERPs.
First of all, you need to choose which URL is the main one and then insert at the top of your preferred URL <head> section:
<link rel=”canonical” href=”http://www.yourdomain.com/your-main-url/” />
Many CMS have integrated that tag and offer solutions to set this up. If not, search engines offer selective use of 301 redirects (if your website displays the same content on http, https and www at the same time), specific URL parameters with Google Webmaster Tools and HTTP headers using PHP or .htaccess.
Thus, there is a few rules to respect if you want your rel canonical to work well:
If you need more insights take a look at this video, where Matt Cutts from Google explains the Canonical link element in detail:
To check your canonicals, you can use OnCrawl. Actually, it will draw a clear view of your canonical performances: if they are matching or not or simply if they are set.
And then, when clicking on a specific segment, you can access full URL details.