Canonical tag: definition, relevance for SEO and practice

A canonical tag tells search engines that a specific URL is the main copy of a page. The use of the tag prevents problems that can arise from identical or similar content under multiple URLs. The canonical tag ultimately defines which version of a URL should appear as the primary variant in the search results.

Canonical tags use a simple and standardised syntax. They are placed in the head section of the respective web page as follows

  • <link rel="canonical" href="example-URL" />

In detail, the components mean the following

  • link rel="canonical": The link in this tag is the main version (canonical version) of the page.
  • href="Example URL:" The canonical version can be found under this URL.

Why is the canonical tag important for SEO?

Duplicate content is a complicated issue. When search engines have to scan and rank many URLs with identical (or very similar) content, this can cause a number of SEO problems:

  • Crawling through a lot of duplicate content can lead to unique top content (unique content) being recognised less.
  • If there is a lot of duplicate content, this has a negative impact on competitiveness in the rankings - the competition with unique content ranks better.
  • And finally, it can happen that search engines rank the wrong URL as the original, meaning that the (SEO) potential of different pages is not realised.

The canonical tag in practice

Many website operators - possibly you too - now assume that they don't need to worry about the canonical tag if they don't actively place content on multiple URLs. The problem is that in the vast majority of cases, duplicate content is created unknowingly or more or less automatically.

For search engines, each unique URL is a separate page that is scanned and evaluated. For example, a homepage can be accessed via both an http and an https URL ( and Or there may be a separate print version of a page with identical content. And mobile-optimised m-variants ( are also perceived separately. There are therefore very quickly numerous copies of a website in play, which should be organised correctly using canonical tags.

Modern content management systems (CMS) and dynamic, code-driven websites exacerbate the problem. Here, tags are often added automatically and multiple paths to the same content are permitted. In extreme cases (especially in e-commerce), hundreds of duplicate URLs can quickly be created without you realising it.

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