Canonical Tag Definition & Best Practices

Canonical tags help search engines differentiate between web pages with very similar or duplicate content, which can negatively affect rankings.

A Guide to Canonical Tags

A canonical tag, also known as rel=canonical, is a part of HTML code that differentiates a master page from similar pages. For example, if you have similar content under different URLs, canonical tags can specify which version is the main piece of content and should be indexed as the original, preferred version by search engines. By showing search engines the master version of content on a site, Google can better understand the content, index it, and determine if it should appear in the SERPs or not.

How Do You Canonicalize a Page?

Creating a canonical tag is relatively easy. However, with the number of URLs in a completed website, canonicalizing pages can be a tedious chore. There are two common ways to canonicalize a page.

Rel=Canonical Directive

Similar to meta tags, they use simple and consistent syntax and are placed in the <head> section of a page:

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

There are two things this code is telling search engines:

  • link rel=”canonical” - The link in this tag is the master version;
  • href=”https://example.com/similar-page/” - This is a similar version.

By using this method, developers won’t have to write multiple redirects for every possible URL, which can be tedious if they are developing a big site. 

301 Redirects

301 redirects can be used when a page has moved to a new location. For example, if a site owner has moved the site to a new domain, a 301 redirect can make the transition as smooth as possible. This practice merges two pieces of content into a single, better performing piece of content, which can ensure any link-building efforts and ranking signals are passed to the new page. It’s important to note that canonical tags allow users to visit both URLs while a 301 redirect does not. For example, if a webmaster uses a 301 redirect from page A to page B, then human users won’t see page A. 

Importance of Canonical Tags in SEO

In terms of search engine optimization (SEO), canonical tags can help preserve a site’s visibility. In order to understand the importance of canonical tags, we must first understand the difference between human readers and bot readers. A homepage, for example, is one page to a human reader. However, the avenues used to get to that page could be different for each user:

  • www.example.com
  • Example.com
  • http://example.com
  • https://www.example.com 

To a human, these look like the same page, and would likely all display the same page when loaded. However, to a bot, these look like four different, unique pages. This is because bots read every URL as its own page. Multiply this challenge across an entire site, and webmasters could be looking at thousands of similar or duplicate URLs. 

Canonical tags can help bots understand the various URLs to a page and correctly identify the main source of content that should be indexed. In the long-run, this can help reduce confusion and increase rankings. Marking your canonical pages can also be important when trying to build links to a site. Site owners can ensure that the right page gets passed authority.

The Risk of Duplicate Content in SEO

Many sites have duplicate content. While there is no Google penalty for duplicate content per se, the search engine does prioritize well-structured content in the SERPs. Additionally, Google also tries to avoid ranking the same domain multiple times in a SERP. So, if a site has three similar pages, search engines might miss the unique content. 


Duplicate content can also affect ranking ability, as search engines don’t necessarily like going through multiple variations of content on one site. If the content does rank, there is a chance that the search engines may have picked the wrong URL as an original. This can affect content marketing strategies if the search engines don’t index URLs correctly. Canonical tags help build a map to show Google which content should be shown in the SERPs.

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When to Use a Canonical Tag

Often, content management systems can automatically add tags, allow multiple URLs to host the same content, and add URL parameters. This can cause many instances of duplicate content throughout a site without the web developer’s knowledge. While it’s good practice to go through a sitemap to clean up canonical tags, there are some proactive measures to take as well. 

Parameter URLs

E-commerce platforms are a good example of sites with very similar URLs. It’s common for URLs to change as customers specify the type of product they want. For instance, the main page of a product could be example.com/product. However, when a customer specifies they’d like a red version of the product, the URL might change to example.com/product?color=red. While humans see this as one page, bots read both URLs as different, unique pages. 

These types of URLs are called parameter URLs, and can be a common cause of duplicate content. Other sites that have this type of content include:

  • Sites with search parameters;
  • Session IDs;
  • Sites with separate printable page versions;
  • And unique URLs for different categories.

Canonical tags can be a vital tool for sites with these types of parameters. 

Mobile-Specific URLs

With 63% of people using mobile for search, webmasters must construct with mobile in mind. Luckily, Google has responded to the spike in new search trends with Accelerated Mobile Pages (AMP). This project was designed so publishers could build web pages that load quickly on mobile devices. 

However, it’s important to use canonical tags to help Google distinguish between main content pages and mobile-specific subdomains. For AMP pages specifically, Google provides guidelines on how to differentiate mobile pages from standard web pages. 

Region or Country-Specific

If a business is global, it’s a good idea to target customers with pages based on their location. This can result in pages with very similar content yet different URLs. Canonical tags should be included if most of the on-page content is similar and in the same language. For instance, if a site has two similar pages in German, the site owner should determine which page should be served to searchers. 

While this practice is similar to using hreflang tags, there is a major difference. Hreflang tags mark pages that are similar in content but are targeted toward an audience that speaks a different language. They are not used to distinguish between duplicate content. Canonical tags can be used with hreflang tags, but not in place of them.

Self-Referential URLs

Sometimes, webmasters will put a self-referencing canonical tag on content that only has one page. While some may see this as redundant, it’s important to understand that, with search engines, it’s better to be safe than sorry. Even if there is only one page of content, the URL can have differentiating factors, like upper or lowercase letters, or could be accompanied by parameters at the end. Canonical tags can help clear the air and make it absolutely clear to search engines which page should be indexed.

Canonical Tag Best Practices

To make the most out of canonical tags, there are some best practices to consider:

  • Use absolute URLs: It’s important to absolute URLs in the rel=canonical link element to ensure the tag is interpreted correctly. For example, webmasters should use <link rel=“canonical” href=“https://example.com/sample-page/” /> instead of this <link rel=“canonical” href=”/sample-page/” />
  • Audit sites for canonicalization issues: Webmasters should inspect for canonical tags. It’s important to ensure that similar or duplicate content has a tag, the tag is pointing to the right page, and the pages are crawlable and indexable.
  • Canonicalize across domains: If a webmaster owns two domains and shares the same content on both, they can use canonical tags to show Google which domain they prefer to show in the search results for that particular piece of content.
  • Use lowercase URLs: Google could treat uppercase and lowercase URLs as two different URLs. Lowercase URLs are easier for human users to search for. It’s important to be consistent in canonical tags to ensure the right URL is being indexed.
  • Use the correct domain version: Despite how it looks, HTTP and HTTPS are not interchangeable. If webmasters use both or one or the other in their canonical tags, this could lead to confusion with search engine bots. 

Canonical tags can help increase SEO efforts and ensure human searchers are finding the correct content. By implementing the correct attributes of the tags and its best practices, webmasters can avoid confusion with search engines and help their website perform as well as possible in search rankings.

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