What is a Canonical Tag?

Rel=canonical, otherwise known as a “canonical tag” is an HTML element that tells search engines which URL is the preferred page. The rel=canonical tag has been supported by most search engines since 2009 and is most often used to prevent duplicate content appearing in results.  When there are multiple versions of the same content, SEORadar recommends using canonical tags.

rel canonical



Why is rel=canonical Important to SEO?

Duplicate content has been an often-discussed SEO issue for years. Unfortunately for site owners, duplicate content can often be the result of the CMS or general nature of the site. For example, ecommerce sites with various product types or color filters may end up with hundreds of URLs for one product. While search engines have become much better at figuring out these types of issues, canonical tags have helped webmasters point search engines in the right direction.



Using rel=canonical Tags

Canonical tags can be used in a number of instances but are most commonly associated with the following:

  • Parameter-based URLs – Most often found with eCommerce sites, parameter-based URLs can be the result of filters, including things like price, rating, color, and more.
  • Character-based URLs – Many CMS systems allow users to use both uppercase and lowercase characters in URLs.
  • Session URLs – Session-based URLs can be generated automatically by your website
  • Country-Specific URLs – Languages, country-codes, etc….
  • Device-Specific URLs – Mobile specific pages and AMP pages
  • Category-Specific URLs – When content is published in multiple categories, some CMS systems create multiple URLs with the category as part of the structure.
  • Other – Trailing slashes, www vs non-www, http vs https, etc.



Benefits of rel=canonical Tags

As canonical tags can be used for a variety of SEO-related issues, it’s no surprise they can have a positive impact on a site’s overall performance. Some of the benefits of rel=canonical implementation include but are not limited to the following:

  • Search Engine Indexing – Index the right page for your users to visit, making it simpler for search engines to know which page to display in search results.
  • Organic Traffic Routed to The Page You Choose – Instead of various copies of the same page vying for ranking, you’ll help search engines to focus on routing organic traffic to one page.
  • Simplify Tracking Metrics – Multiple URLs for the same product or content make reporting more challenging to consolidate.
  • Manage Syndicated Content – Consolidate page importance when you have duplicate content across multiple domains to your preferred domain. 



Rel=Canonical Best Practices

  1. Pick the most important page as your canonical.  If there are multiple pages that are equally important, pick the one with the most traffic or links.  You could also choose the simplest version of the URL (for example, the one without parameters or session ID).  Don’t mix up canonicals where page A -> B and page B -> C.  Simplify A->C and B->C.
  2. Use a self referring canonical, especially on the home page, which is ideally setup as a 301 redirected to the preferred www/non-www version (or http/https version). Go ahead and set the canonical. 
  3. Don’t confuse 301s with canonicals.  301s are for permanent redirects and should be used in situations such as where http pages are redirected to https pages.  Canonicals are best suited for duplicate content shown on multiple URLs that you still want active.
  4. Make sure you don’t noindex the canonical version of a URL
  5. Set the canonical using the full URL not the relative URL
  6. Set only one canonical on a given page.  



Rel=Canonical Tag vs. Other Methods

Canonicals are often confusing.  Whether it’s a 301 or something more advanced, there are various instances that create confusion.  Often it is challenging to know when to use which strategy.  Below is a comparison of the different ways to canonicalize multiple URLs vs other methods.

  • Canonical vs. 301 Redirect – In some cases, a 301 redirect isn’t an option (ex. E-commerce page with multiple color variations of the product) and using a canonical can have the same impact.  When there is no reason to have duplicate content (ex. www-version of a home page and a non-www version or https and http versions of the same page), then a 301 is preferred since it tells the search engine that all traffic should be redirected to the preferred page.
  • Canonical vs. Passive Parameters – Passive parameters won’t help search engines identify the best URL to drive traffic to and will often add difficulty for search engines to decide the URL that best matches a search query. Setting canonicals for all the parameters to route back to the preferred page will help search engines to focus on certain URLs, but it can also use significant crawl budget.  Sites that have a large number of passive parameters, some choose to block bots from crawling them with a robots.txt directive (ex: Disallow:/*?tag=*).  This can be tricky since some parameters are used for tracking metrics.  Make sure to link to the canonical version and not a parameter version of any page.
  • Canonical vs. Location Hashes – Some sites use location hashes or URL fragments like parameters.  This can work to have search engines focus on the non-hash version of the URL since Google ignores URL fragments or location hashes (#ScrollsUserToDifferentPartOfPage) when it comes to indexing. But if you’re trying to get the hashed version indexed, you may miss out.  If you’re trying to index 2 parts of a page separately, location hashes won’t work to do that.  It’s best to have 2 distinct URLs with the content separately.  Location hashes simply scroll the user to different parts of a page.



Identifying Changes with Canonical Tags

SEORadar monitors URLs for changes to canonical tags so that you know when issues arise.  Whether it’s a weekly code change to improve the UX or features of the website or a major site migration, rel=canonicals seem to not be well understood by each person that works on large and small sites.  By tracking SEO changes, you’ll know when a variety of issues appear.

  • Linking to an invalid URL  – the canonical will not be respected
  • Linking to the wrong URL – the canonical will not be respected
  • Linking to a content that is not true duplicate – the canonical may or may not be respected
  • Canonical tag is removed – duplicate content may dilute search ranking results
  • Canonical tag appears – the canonical will be respected
  • Self referential gets deleted -duplicate content may dilute search ranking results
  • Link to a page that is non-indexed – page may be removed from search rankings
  • Linking to a page that is blocked by robots.txt – page may be removed from search rankings



Common Questions About rel=canonical Tags

  • What are the challenges in implementing canonical tags?

There are a variety of challenges, including choosing the singular URL to become the canonical, knowing when to use canonical tags vs. 301 redirects, and maintaining canonical tags. 

  • Where should the canonical tag be placed?

Canonical tags should be placed within the <head> section of the HTML.

  • How can you test if canonical tags are implemented properly?

Most site audit tools can test canonical tags.  Remember, these site audits usually only test once. Continued monitoring via a service, such as SEORadar, can ensure that canonical tags stay implemented properly.


Future of rel=canonical Tags

Canonical tags have been helpful in giving SEO managers control over which page is the most important page of duplicates.  For now, it’s the easiest way for crawlers to better understand complex sites. 


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