rel=canonical: the ultimate guide

A canonical URL lets you tell search engines that certain similar URLs are actually the same. Because sometimes you have products or content that you can find on multiple URLs — or even multiple websites. Using canonical URLs (HTML link tags with the attribute rel=canonical), you can have these on your site without harming your rankings. In this ultimate guide, we’ll discuss what canonical URLs are, when to use them and how to prevent or fix a few common mistakes!

What is the canonical link element?

The rel=canonical element, often called the “canonical link” or “canonical tag”, is an HTML element that helps webmasters prevent duplicate content issues. It does so by specifying the canonical URL, the preferred version of a web page. Most of the time, this is the source URL. Using canonical URLs is an important part of improving your site’s SEO.

The idea is simple: If you have several versions of the same content, you pick one “canonical” version and point the search engines at that one. Adding the canonical element to a URL tells search engines that’s the one they should show in their results.

For example, the canonical URL for our WordPress SEO article looks like this:

<link rel="canonical" href="https://yoast.com/wordpress-seo/" />

A canonical URL can be seen in the source of a webpage, by searching for rel="canonical". It’s an element only the search engines see, your users won’t be affected by it.

The SEO benefit of rel=canonical

The history of rel=canonical

The canonical link element was introduced by Google, Bing, and Yahoo! in February 2009. If you’re interested in its history, we recommend Matt Cutts’ post from 2009. Or watch the video of Matt introducing the canonical link element. Because, although the idea is simple, the specifics of using it are often a bit more complex.

As we mentioned above, a canonical URL is a technical solution for managing duplicate content. Duplicate content can be a big problem for SEO. When search engines find two (or more) very similar pages, they don’t know which one to add to the search results. As a result, all of those pages might end up ranking lower.

You might, for instance, have a post or product that is attached to two categories and exists under two URLs, like so:

https://example.com/black-shoes/black-and-red-shoes/
https://example.com/red-shoes/black-and-red-shoes/

If these URLs are both for the same product, setting one as the canonical URL tells search engines which one to show in the search results.

You can also use canonicals to point search engines to the original version of an article. For instance, if you’ve written a guest post for another website. If you want to post it on your site too, you could agree to post it with a canonical link to the original version.

The process of canonicalization

When you have several choices for a product’s URL, canonicalization is the process of picking one of them. Luckily, it will be obvious in many cases: one URL will be a better choice than the others. But in some cases, it might not be as obvious. This is nothing to worry about. Even then, just pick one! Not canonicalizing your URLs is always worse than canonicalizing the “wrong one”.

How to set canonical URLs

Ironic side note

The term ‘Canonical’ comes from the Roman Catholic tradition, where a list of sacred books was created and accepted as genuine and named the canonical Gospels of the New Testament. The irony is it took the Roman Catholic church about 300 years and numerous fights to come up with the canonical list, and they eventually chose four versions of the same story…

Let’s assume you have two versions of the same page, each with exactly – 100% – the same content. The only difference is that they’re in separate sections of your site. And because of that, the background color and the active menu item are different – but that’s it. Both versions have been linked from other sites, so the content itself is clearly valuable. So which version should search engines show in the results?

The situation described above occurs pretty often, especially in a lot of e-commerce systems. A product can have several different URLs depending on how you got there. But this is precisely what rel=canonical was invented for. In this case, you would apply rel=canonical as follows:

1. Select which page will get the canonical tag

Say, for example, you have these two URLS, both showing identical content:

https://example.com/wordpress/seo-plugin/
https://example.com/wordpress/plugins/seo/

Pick one of your two pages as the canonical version. This should be the version you think is the most important. If you don’t care, pick the one with the most links or visitors. When all these factors are equal, flip a coin. You need to choose.

2. Add the rel=canonical element

The next step is to add a rel=canonical link from the non-canonical page to the canonical one. Doing that manually means adding rel=canonical as a meta tag in the HTML header of the non-canonical page. You can also use the canonical tag feature in Yoast SEO (we’ll talk more about that a bit later on).

So, if we picked the shortest URL as our canonical URL, the other URL would link to the shortest URL in the <head> section of the page – like this:

<link rel="canonical" href="https://example.com/wordpress/seo-plugin/" />

It’s as easy as that! Nothing more, nothing less.

What this does is “merge” the two pages into one from a search engine’s perspective. It’s a “soft redirect” without actually redirecting the user. Links to both URLs now count as the single, canonical version of the URL. Helping your page(s) reach its full ranking potential.

Want to know more about the use of rel=canonical on category and product pages of your ecommerce site? We also discuss this topic in this Ask Yoast video.

Setting the canonical URL in Yoast SEO

With the canonical URL tag feature of the Yoast SEO plugin, you can easily add a canonical URL to a post or page. You only need to do this if you want to change the canonical to something different from the current page’s URL. Yoast SEO already renders the correct canonical URL for almost any page type in a WordPress install.

For posts, pages, and custom post types, you can edit the canonical URL in the advanced tab of the Yoast SEO metabox — or in the sidebar of the WordPress block editor:

For categories, tags, and other taxonomy terms, you can change the canonical URL in the same place in the Yoast SEO metabox or in the sidebar. If you have other advanced use cases, you can also use the wpseo_canonical filter to change the Yoast SEO output. You can read more on our help page about using Yoast SEO to set canonical URLs.

When should you use canonical URLs?

301 redirect or canonical?

If you are unsure whether to do a 301 redirect or set a canonical, what should you do? The answer is simple: you should always do a redirect unless there are technical reasons not to. If you can’t redirect because that would harm the user experience or be otherwise problematic, set a canonical URL. The redirect manager in Yoast SEO Premium helps you set up redirects.

Should a page have a self-referencing canonical URL?

In the image above, we link the non-canonical page to the canonical version. But should a page set a rel=canonical for itself? This question is a much-debated topic amongst SEOs. At Yoast, we strongly recommend having a canonical link element on every page, and Google has confirmed that’s best. That’s because most CMSs will allow URL parameters without changing the content. So all of these URLs would show the same content:

• https://example.com/wordpress/seo-plugin/
• https://example.com/wordpress/seo-plugin/?isnt=it-awesome
• https://example.com/wordpress/seo-plugin/?cmpgn=twitter
• https://example.com/wordpress/seo-plugin/?cmpgn=facebook

The issue is that if you don’t have a self-referencing canonical on the page that points to the cleanest version of the URL, you risk being hit by this. And if you don’t do it yourself, someone else could do it to you and cause a duplicate content issue. So adding a self-referencing canonical to URLs across your site is a good “defensive” SEO move. Luckily, our Yoast SEO plugin takes care of this for you.

Cross-domain canonical URLs

Perhaps you have the same piece of content on several domains. Some sites or blogs republish articles from other websites independently, as they feel the content is relevant for their users. In the past, we’ve had websites republishing articles from Yoast.com as well (with express permission).

But if you had looked at the HTML of every one of those articles, you’d found a rel=canonical link pointing right back to our original article. This means all the links pointing to their version of the article count towards the ranking of our canonical version. They get to use our content to please their audience, and we get a clear benefit from it. This way, everybody wins!

Faulty canonical URLs: common issues

There are many examples out there of how a wrong rel=canonical implementation can lead to huge issues. We’ve seen several sites where the canonical on their homepage pointed at an article, only to see their home page disappear from search results. But that’s not all. There are other things you should never do with rel=canonical. Here are the most important ones:

• Don’t canonicalize a paginated archive to page 1. The rel=canonical on page 2 should point to page 2. If you point it to page 1, search engines will actually not index the links on those deeper archive pages.

• Make them 100% specific. For various reasons, many sites use protocol-relative links, meaning they leave the http / https bit from their URLs. Don’t do this for your canonicals. You have a preference, so show it.

• Base your canonical on the request URL. If you use variables like the domain or request URL used to access the current page while generating your canonical, you’re doing it wrong. Your content should be aware of its own URLs. Otherwise, you could still have the same piece of content on – for instance – example.com and www.example.com and have each of them canonicalize to themselves.

• Multiple rel=canonical links on a page cause havoc. When we encounter this in WordPress plugins, we try to reach out to the developer doing it and teach them not to, but it still happens. And when it does, the results are wholly unpredictable.

rel=canonical and social networks

Facebook and Twitter honor rel=canonical too, and this might lead to weird situations. If you share a URL on Facebook with a canonical pointing elsewhere, Facebook will share the details from the canonical URL. In fact, if you add a ‘like’ button on a page that has a canonical pointing elsewhere, it will show the like count for the canonical URL, not for the current URL. Twitter works in the same way. So be aware of this when sharing URLs or when using these buttons.

Advanced uses of rel=canonical

Canonical link HTTP header

Google also supports a canonical link HTTP header. The header looks like this:

Link: <https://www.example.com/white-paper.pdf>;    rel="canonical" 

Canonical link HTTP headers can be very useful when canonicalizing files like PDFs, so it’s good to know that the option exists.

Using rel=canonical on not-so-similar pages

While we wouldn’t recommend this, you can use rel=canonical very aggressively. Google honors it to an almost ridiculous extent, where you can canonicalize a very different piece of content to another piece of content. However, if Google does catch you doing this, it will stop trusting your site’s canonicals. And that’s bad news for your SEO.

Using rel=canonical in combination with hreflang

We also talk about canonicals in our ultimate guide to hreflang. That’s because it’s essential that when you use hreflang, each language’s canonical points to itself. Ensure that you understand how to use canonicals well when you’re implementing hreflang; otherwise, you might kill your entire hreflang implementation.

Conclusion: rel=canonical is a power tool

Rel=canonical is a powerful tool in your SEO toolbox. Especially for larger sites, the process of canonicalization can be critical and lead to major SEO improvements. But like with any power tool, you should use it wisely as it’s easy to cut yourself. We hope this guide helps you understand this powerful tool and how (and when) you can use it.

Read more: WordPress SEO: The definitive guide to higher rankings »