What is canonicalization?

Canonicalization is the process of defining a dominant version or ‘canonical’ version of a page and redirecting all other URL’s with the same content to a default version of that page. Without canonicalization in place, Google will crawl this content on separate URLs and consider it duplicate content. By using canonicalization for SEO, you control which version of your page will get the link equity and rank in Google.

Reasons for canonicalization

Canonicalization is an important SEO best practise because:

  • If there are multiple versions of a page with the same content, Google and other search engines won’t be able to identify which version to include or exclude from the search results
  • If the same content can be found across multiple URL’s, search engines are unable to identify which source was the original
  • With multiple URL’s containing the same content, search engines would not know where to direct link metrics such as trust and link equity to if users are linking to multiple versions of the same page

When canonicalization is needed

Canonicalization should be used in the following scenarios:

  • Session IDs or tracking codes are automatically added to your URLs i.e. http://www.example.com/category/SESSID=459
  • When users filter information on your webpages, the URL changes to meet their filter requirements i.e. http://www.example.com/seo-services?type=management
  • You have multiple versions of your homepage i.e.
    • example.com
    • http://example.com
    • http://www.example.com/
    • http://www.example.com/index.php

How to use the canonical URL tag

To create a canonical version of a page that has multiple URL’s, you should add the following code into the header of all of the ‘duplicate’ pages besides the ‘default’ page that you wish to be the canonical version which will be the URL to appear in the search results as well as retain all the ranking and link equity benefits.

<link rel=”canonical” href=”http://example.com/blog” />

This code tells search engines (not users) that this URL should be viewed as a copy of the canonical URL and any metrics should go to the canonical URL. In a sense, the canonical tag is much like a 301 redirect without the redirection. To follow on from the previous example of having multiple versions of your homepage, you would add the canonical tag to the following pages:

http://www.example.com/index.php
http://example.com
www.example.com

By adding the canonical tag to each of the homepage URL’s besides the one you want to be the canonical, you are telling search engines that http://www.example.com/ is the canonical URL. The canonical tag would be:

<link rel=”canonical” href=”http://www.example.com/” />