What is a canonical URL? A guide to SEO

The phrase “canonical tag” comes up a lot in SEO discussions, and probably more so when working with cross-functional partners like engineering, analytics, etc.

At first glance, this might seem a bit daunting, especially if you’re new to the SEO space.

Good news, though: with this guide, we’ll go through the fine print around what a canonical tag is, why it’s important, what it looks like in the wild, where it belongs, and some practical details.

What is a canonical?

First of all, before you even define a canonical tag, if there’s one piece of information you take from this guide, let it be this: canonical tags are no guidelines like the Robots.txt file.

This means that Google considers canonical tags as a strong clue, but ultimately it considers many signals and decides whether to honor them or not.

Now that we have the golden rule out of the way, let’s see what it is!

The canonical tag came into play in 2009 as an HTML tag found in source code to tell search engines which URL is the primary version of a page. This can be leveraged to tell Google which page variant to index for users.

A canonical tag is the HTML tag itself on a page, but the “canonical” – now that’s a bit different.

There are two easy ways to define canonical variations: a user-declared canonical version and a Google-declared canonical version.

  • Declared by user canonical: This is precisely what he says; this is the canonical specified in the canonical tag.
  • Canonical declared by Google: This is the URL that Google chooses to honor as the canonical URL.

If you have access to Google Search Console, you can take advantage of the URL Inspection Tool to see the two canonical types above.

In ideal condition, they match – but what if they don’t? We’ll talk about that later.

Screenshot of Google Search Console Inspect URL, October 2022

How Google Chooses a Canonical URL

When Google crawls and indexes a site, it examines the main content of the page. (Pro tip: don’t confuse content with written-only content).

In this analysis, it will likely discover similar pages, and Google will then choose the page it thinks is the best representation of what the page is trying to convey to users and select it as canonical.

As we saw above, a canonical tag is not a directive, so in addition to the canonical tag itself, Google considers other signals – so be consistent!

Internal links and external links are just some of the additional factors considered by Google with a canonical tag.

Caution: If you internally link your pages with query parameters like /?some_parameter=xyzchances are that Google will ignore your canonical meta tag and choose a URL with a query parameter as canonical.

Google scans RSS feeds very aggressively, so make sure your declared canonical matches match your RSS feed URLs.

If you add URLs to your RSS feed with parameters like /?source=stream in order to track traffic to your site from RSS subscribers, Google may choose a canonical with the query string e /?source=stream even if it’s a tracking parameter – and Google knows that.

You can use link shortening services for your URLs in the RSS feed so you can track clicks on them or use RSS services such as FeedPress.

Google will also make choices for the sake of the user experience.

If you have a desktop version of your site, Google may offer the mobile version to a user on a mobile device.

How Canonical Tags Can Be Helpful For SEO

Canonical tags are essential for sites with a handful of pages and millions of pages.

They are needed for several reasons.

1. You choose the canonical tag

The canonical tag is an opportunity for you to offer Google the very best version of a page on your site that you want to offer to Internet users.

2. Duplicate Content

Duplicate content is one of those areas that seems simple on the surface but is more complicated than its name suggests, and tends to carry a negative connotation with it.

So you might be thinking “I don’t have duplicate pages”, but before making that statement, let’s take a look at what can be defined as “duplicate” via Google Search Central Documentation.

Duplicate pages can be categorized as all pages containing the same main content in the same language. Suppose you use different pages to support mobile pages (an m., amp, etc.) and dynamic URLs that help things like parameters or session IDs.

In this case, your blog creates paths in several folders; you have an HTTP and HTTPS version of your site, and your site has duplicate content. It’s not a panic and it’s quite common, hence the canonical significance!

3. Google uses canonicals as the primary source

Google uses the canonical to determine the content and quality of a page.

Canonical pages are crawled more frequently than non-canonical pages.

4. Can help with crawl budget

You’ve probably heard a lot about the phrase “crawl budget” if you have a relatively large site.

When done correctly, canonicals can help ease the burden on your crawl budget because Google will crawl canonical versions of pages much more frequently than the non-canonical version.

It does not replace tags without indexes, redirects or a robots directive.

5. Consolidate Bonding Signals

Canonicals guide search engines to take various pieces of information they have for multiple similar pages and consolidate them into a single URL, increasing its value.

6. Content Syndication

If you have a site that syndicates its content for publication or is operated by partners, you want to make sure that’s your version that shows up in search results.

How to implement a canonical tag

Now that we’ve gone over the what and why of canonical tags, let’s talk a bit more about how to implement a canonical tag on your site.

Pages can (and should) have self-referencing canonicals when they are the best version of the page to consolidate things like metrics tracking, HTTPS versions, mobile experiences, etc.

Unless you can modify the HTML code directly, you will probably need to work with your development/engineering partners.

A canonical tag is a line of code that you add to the section

from any page.

It may look like this:

HTML canonical tag exampleScreenshot by author, October 2022

Canonical Tags FAQ

Q: Can I use Canonical on multiple domains?

A: Yes, you absolutely can. For example, if you have a variety of sites where you post the same article on different websites, using a canonical tag will focus all the power on the version you have selected as canonical. This would also be good practice for syndicated content best practices when working with sites you don’t own.

Q: Do canonical tags convey link equity?

A: The consensus is yes, they do, but canonicals should not be confused with the same as a 301 redirect.

Q: Should I use a canonical tag or a tag without an index?

A: First and foremost, a tag without an index is a directive, unlike a canonical tag intended to remove a page from the index. A canonical tag is a great solution when you want to group all related links and signals into a single URL.

Our favorite answer in the SEO space applies to this question, “it depends”. In a SEJ where John Mueller discusses when to use a canonical or a noindex, it details a little more the questions to be asked to choose one rather than the other or… both.

Q: Should I use a 301 redirect or a canonical tag?

A: A 301, like a tag without an index, is a directive. This is another “it depends” situation; however, there are some things to consider when choosing one over the other. If you have two very similar pages and you don’t need both to be online for business reasons, a 301 redirect might be a good choice.

A good example would be a product page that is permanently out of stock or an old page that is no longer worth updating. You can read more use case scenarios in This article which goes into detail around 301 versus canonical tags.

Q: What happens if Google does not respect the chosen canonical?

A: As mentioned above, there may be times when Google does not respect the canonical you have chosen, and you can view this information through the URL Inspection tool in Google Search Console. There can be several reasons why Google does not respect the canonical selected by the user.

The tag may not be correctly implemented; the site signals contradict the selected canonical possibilities and various other possibilities. It is likely that you will need to perform a scan to determine the root cause.

Conclusion

We hope this guide has helped you understand the what, where, and why of using canonical tags. Be sure to review your canonical tags and see where improvements can be made to help get your favorite information noticed by search engines.

More resources:


Feature image: Luis Molinero/Shutterstock

About Nereida Nystrom

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