Everything you need to know about the canonical tag
Posted by Owen Powis on
The canonical tag is a way to pass value from one page to another without using a redirect. ‘Duplicate content’ has become a widespread issue and is being increasingly targeted by search engines. Prevent the damage it can do to your site with the canonical tag. This article covers the basics on what it is and ways to successfully implement it.
What the canonical tag is
Duplicate content is where two or more pages have the same content. The canonical tag is a piece of HTML code which can be placed on duplicate pages to indicate to a search engine where the original version of that page is. It looks like this:
<link rel = “canonical” href= “http://www.domain.com/originalcontent” />
It is placed within the <head> section of a page, ie, the section before the <body> tag. If you don’t know what these are, look at the code behind this web page and search for them. You can do this by right clicking on the page and selecting 'View source'. Then press ‘ctrl + f’ ('cmd + f' on a Mac) to search for them. You will see that these tags hold the different types of information contained by the page.
Canonical tag behavior
Search engines do not want the same content listed multiple times in the search results. Each result should give different content. This means where multiple pages with the same content are found, Google will choose to attribute just one of them with value. This means only one will rank in the results.
A canonical tag shows, when the page is a duplicate, where the search engines can then choose to attribute the value of the page to. In order to do this the search engine will need to visit both pages and compare them.
The canonical is an instruction, telling the search engines where they can find the original version. Unlike a redirect it does not have to be followed and sometimes search engines will choose to ignore it. If the pages involved have different content it is unlikely to work.
Where it should be used
The canonical tag can be an ideal candidate in a couple of scenarios, both of which revolve around duplicate content. The first is where you have lots of internal duplicate content. This happens commonly on more complex sites.
One page may be found in a variety of ways: for example if the page is found by clicking through the site's navigation the URL may be different than if that same page is found through using the site search:
The page below has different URLs depending on how you access it.
Found through navigation:
http://www.johnlewis.com/328988/Product.aspx
Through a product Search: http://www.johnlewis.com/328988/Product.aspx?SearchTerm=Fenn+Wright+Manson+Long+Cardigan%2c+Khaki
Or where the URL is being used as part of campaign tracking. This is from the email campaign of a different site:
http://www.next.co.uk/sale/?utm_source=Adhoc&utm_medium=Email&utm_campaign=SS12_EOSS
And the same page when accessed from within the site:
http://www.next.co.uk/sale/#LID=01_03_01_03
But there’s also a clean canonical URL:
http://www.next.co.uk/sale/
So if you google ‘Next Sale’...
It’s the clean URL that comes up. The canonical tag has been put to good use. Simply put, if you have multiple versions of the same page on different URLs then the canonical tag is the solution.
The second scenario where you may want to implement it is on a page by page basis and utilize its ability to work across domains. So let’s say you own two news-focused domains and sometimes like to repeat a story on one that was successful on the other. A redirect might confuse readers being taken to a new property and just linking to the story with a summary isn’t ideal.
The canonical tag can get around this, allowing you to show the content and attribute it back to the original source as well. Meaning that your readers don’t need to leave your site to see the content and you won't get a duplicate content penalty.
Summary
A canonical tag ...
- Should be used instead of a redirect to clear up duplicate content.
- Can be used across domains.
- Will only work where content is very similar.
- Is an instruction not a command.
Further reading and resources
http://support.google.com/webmasters/bin/answer.py?hl=en&answer=139394
http://yoast.com/wordpress/seo/
http://en.wikipedia.org/wiki/Canonical_link_element
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About Owen Powis
Owen Powis is the lead SEO Consultant at Wordtracker and has a half decade of experience working as an SEO Consultant. That time has been spent working at some of the UK's largest agencies dealing with clients ranging from SMEs to large blue chip organizations. He can be found @owenfantastic on Twitter.
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