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The Role of Canonical URLs in SEO

Posted 09th February, 2015 by Aliysa

As your website grows, you may find yourself with copies of the same content in multiple places. It's a perfectly normal practice, but it does provide search engine bots with a (metaphorical) headache: which page should they show in their results pages? For the end user's sake, search engines try to avoid ranking content twice, so bots are forced to decide which page holds greater preference, via a process called 'canonicalisation'.


When do Pages need to be Canonicalised?:

Search engines expect some overlap between different pages on your site (contact address, taglines etc), however pages with a copied paragraph or article cause strong conflict for bots. The most common examples noted by Google, Bing and our own support team are:

  • Blog posts stored under multiple categories: because of how many CMSs work, some blog posts can be found at two separate URLs (ie /blog/category-1/title and /blog/category-2/title). It's therefore unclear which should gain ranking power.
  • Dynamic page content: your page serves different content depending on your user sessions. A great example is our domains search page, which brings up personalised results when you enter a query.
  • Your server configuration: if your SSL certificate isn't properly installed, search bots may be conflicted as to whether you want to serve the http:// or https:// version of your site. In such cases bots may choose to canonicalise the http:// option, which is damaging for SEO.

Search bots will attempt to pick a canonical URL based on which content it thinks is the original and which page your internal links point to.


The Potential Problems of Canonicalisation:

Canonicalisation isn't completely fool-proof, take a look at these three URLs for instance:

Although you might consider them to be the same, they're actually different URLs and a web host could return different content for each. Therefore if you fail to point all your internal links to your preferred URL then search bots will split your ranking power between every URL variation that it finds when trawling your site. Consiquently your page will actually be ranked multiple times, further down the search engine results page (SERP), causing a huge SEO headache.

That might not sound like the end of the world, but remember that over 70% of Google searches never get past page 1, so any division of ranking power will massively slash your exposure to users via SERP.


Preventing an SEO Disaster:

It's really easy to stop search bots from carving up your search ranking, in fact all you need is one simple line of code within the tags of each of your unpreferred pages:

<link rel="canonical" href="[canonical URL]" />

Note: in the above code replace the text '[canonical URL]' with your actual canonical URL address.

This tells search engines that your page contains copied elements and the ranking power should instead be delivered to your canonical page.

Alternatively you can fix the problem by adding a 301 redirect to each of your copied pages, diverting users to the original. Google claim to slightly prefer this method of canonicalising pages, but it's not always appropriate for your content so I'd suggest only using it when you're certain that it's advantageous. A 302 will perform the same function but these redirects don't pass any ranking power onto your preferred page, so it's better to avoid this option.


As your website grows in complexity, it becomes naturally harder to avoid search engine penalties, and in many cases webmasters don't actually realise that non canonicalised URLs are holding back their SEO efforts. This is one of those small details that's very easy to overlook, but could actually cause substantial damage in the long run.

Categories: Tips, SEO

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