Definition
The canonical URL is a tag used to indicate to a search engine the “main” URL to be taken into account for indexing, when the same content is present on different URLs. So, with this tag, you prevent the engine choosing the URL for you.
What is it for?
The canonical tag plays a role in duplicate content cases, in particular in the following cases:
Intentional duplicate content: you need the content to be available on two different pages. For example, you sell a product in different colors, but you’ve got no interest in creating different contents to differentiate the pages. In this case, you have to choose the product that sells the best, or the one that has the best traffic, and put a canonical tag in the duplicated page’s <HEAD> to the main "canonical" page. Then, it will be the latter which will be indexed:
<link rel="canonical" href="https://www.website.com/canonical-page>
Unintentional duplicate content: to avoid duplicate content, or unintentional duplicate URLs following a bug, you must indicate a canonical tag, this time in the canonical page to point to itself.
==> Conclusion: each website’s page must have a canonical tag.
If the page is duplicated: the tag points to the canonical URL.
If the page is canonical: the tag points to itself.
Do you want to check and analyze canonical URLs? Don’t hesitate to check out the method sheet.
How is it different from a 301 redirect?
The canonical tag is very useful to avoid duplicate content, but be careful: it does not replace the 301 redirect.
Indeed, if you have no interest in having duplicate content on a website, then redirect the duplicate versions 301 to the canonical version rather than a simple canonical tag.
For example, you’ve got a technical bug that generates duplicate URLs: https://mywebsite.com/ and https://mywebsite.com.
In that case, choose a URL version, with or without the "/" at the end, and set up a 301 redirect from the duplicated version to the correct one.
Note: this is a case of Duplicate URL Same Text (or DUST) which designates the identical content’s presence under 2 different URLs.
The most frequent errors
Here is a non-exhaustive list of the errors that are most often found when setting up canonical URLs:
Put the relative URLs. It’s better to put absolute URLs, so with http:// or https://
Specify different canonical URLs for the same page
Real and canonical URLs different from a slash (/) at the end
Canonical URLs’ inconsistent mix and redirects: A indicates B as canonical, but B redirects to A
It's your turn to set up your canonical tags! 😊
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