Canonical Tag

The Canonical Tag identifies the preferred (canonical) URL for duplicate content. This way, search engines know which version should be indexed – e.g., with or without www, parameters, or print versions. Alternatively, redirects help.

In 2009, the Canonical Tag was introduced – why? One of the biggest challenges for search engines is combating duplicate content. Identical or similar content on websites not only causes search engines like Google to crawl unnecessarily many pages but also results in indexed pages that provide no added value to users. Consequently, search results worsen, which is contrary to the goals of search engines: to provide users with the best possible results for their queries. Therefore, especially Google places high value on unique content as part of modern search engine optimization. The Canonical Tag provides a remedy, as it is an HTML indicator in a webpage's source code that points to original content or duplicate content. As a result, the search engine crawler knows that the canonical (preferred) URL is to be considered the original source and indexed for content. However, there is no guarantee that the preferred URL will be indexed, even though search engines generally follow this indication that a URL is canonical.

Applications of Canonical Tags

The canonical tag is used against duplicate content, which requires knowledge of identical or similar content. This leads to different possibilities or situations where the canonical tag should be set. Many websites can be accessed via different URLs (e.g., with and without www.), which consequently leads to duplicate content. Similarly, URLs with and without capitalization or session IDs can cause the problem of duplicate content. Additionally, different versions of pages, such as print versions or PDFs, should be considered. Press releases published on numerous PR portals on the internet, while also being content on one's own website, are also problematic.

The specification of a canonical URL is usually done in the head section of a webpage's source code, so that search engine crawlers recognize the reference as the original right at the start of the crawling process. The canonical tag consists of a link attribute and the reference rel="canonical" and looks like this in the source code: <link rel="canonical" href="http://www.example.com/ExamplePage"/>. In this example, the entire URL is specified (with www.), which is preferred by search engines and should be the ideal case. Otherwise, misunderstandings or errors may occur during the indexing of pages.

When a Canonical Tag is not helpful

Canonical tags are important for indicating that a URL is canonical, but it is not always sensible to set a canonical tag or the reference does not solve the problem in every situation. This is especially true for content theft. In this case, the entire website is copied and content is displayed under a different URL. In this case, duplicate content is created, which cannot be easily removed by webmasters, as the canonical tag cannot be implemented on the foreign site.

Alternative to Canonical Tags: Links with Redirect

An alternative to canonical tags are links that perform a redirect. Especially when there are several synonyms for a keyword that all generate substantial search queries, such a redirect to an appropriately optimized page is advisable. This is a better solution than creating multiple categories that offer the same products. In such cases, a canonical tag is not necessarily needed.