What Are Canonical Tags, Why Are They Important for SEO?

October 16, 2020

Kerrie Ashall

Last week, we posted about the canonicalisation and mobile-indexing issues Google was experiencing as of late September. This covered the recent issue of syndicate content being incorrectly canonicalised and web pages losing organic traffic and ranking lower in the SERPs. This has marked the question of just how relevant canonicalsation was to SEO.

As a follow-up, we will take a deeper look into the subject of canonicalisation, specifically, what is a canonical tag, why do we use them in the SEO community, and how do we implement them?

What is a canonical tag?

A canonical tag (also referred to as a canonical link or canonical URL) is included in the HTML source code of a webpage. It is used to differentiate duplicate content from its origins by notifying Google which URL is the ‘original’ or ‘master copy’. They can be found in the <head> section of a web page’s HTML source code (see example):

<link rel=”canonical” href=”https://www.website.com/page/” />

Why do we use them?

Canonical tags are an essential part of a successful SEO strategy, as they ensure that the original page is credited and prevent any duplicate content penalties from Google.

Search engines don’t like duplicate content – they will only index one version of a page – and canonical tags help solve this issue by telling the search engine which URL you want to rank in the search results.

Google Search Liaison’s Danny Sullivan previously mentioned the importance of canonicalisation when it comes to republishing content;

In our previous article – where we mentioned syndicated content – we talked about site owners allowing their content to be republished on another site. This can be a good strategy for helping your content reach a wider audience.

However, this is not the only example of duplicating content, at least not in Google’s eyes. If you have a page that can be reached via multiple URLs, or different pages that share similar content, then Google will also view these as duplicates. Google will then decide which URL is the original, or canonical version, and will crawl that page, while the others will be seen as copies. So, unless you make it clear which URL is canonical, Google will simply decide for itself.

How do we implement a canonical tag?

These are some of the ways that you can implement a canonical tag:

  1. HTMLtag (rel=canonical)

This is the most straightforward method: just add the following code to the <head> section of any duplicate page:

<link rel=“canonical” href=“https://example.com/canonical-page/” />

  1. HTTPheader

For PDF documents, there’s no page <head> section, so you can’t put canonical tags in the header. Instead, use HTTP headers to set canonicals.

  1. 301 redirect

When you want to lead traffic away from a duplicate URL and to the canonical version, use 301 redirects. If your page is accessible at several URLs, choose one URL as the canonical version and simply redirect the others there. Do the same for https/http and www/non-www versions of your site, too.

Things To Avoid

Although canonical tags are evidently useful, especially if you are republishing older content, there are some things you need to avoid when using them. For instance, setting your site’s homepage as the ‘master’ URL; this may seem reasonable at first, but if all your canonicalised pages lead back to your homepage, then you could end up having none of your pages indexed by Google.

And Google will ignore any canonical tags that are linked to content that isn’t actually a duplicate; for example, if a site owner is trying to pass off a page as a duplicate so that it can be canonicalised and they can then rank higher in the SERPs. So, only use canonical tags for duplicated or closely matching content.

Also, do not use multiple canonical tags; pages can only have one canonical tag, otherwise they won’t be indexed.


As mentioned before, Google doesn’t like duplicate content. This doesn’t mean it is a bad thing to have duplicate content, as many site owners use it as an SEO tactic to boost their rankings. However, if you do have any duplicate content, you need to make sure you’re not missing out. You should decide on the best way to implement canonical tags so that you don’t lose out on any organic traffic. Don’t let Google decide for you.






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