A canonical URL is a way for search engines to indicate that a particular URL represents a master copy of a page. The use of the canonical tag prevents problems that arise due to identical or “duplicate” content appearing on multiple URLs. Practically speaking, a canonical URL tells search engines which version of the URL you want to appear in search results.

This is a technical solution for duplicate content. For example, you might have a post or product that is related to two categories and exists under two URLs.

Canonical URL and duplicate content

Duplicate content is an important topic for SEO, and therefore it is an important item when it comes to building and designing websites. When search engines search for a lot of URLs with identical (or very similar) content, it can cause a number of SEO problems. First, if search engines have to go through too many duplicates, they may miss out on some unique content. Secondly, the existence of duplicate content can reduce the possibility of ranking. Even if the content ranks, search engines may choose the wrong URL as “original.” The canonical URL helps control duplicate content.

For search engines, each unique URL represents a separate page. Given the many variations of URLs that can occur and the number of “same” pages that search engines recognize, it can be quite large. Modern content management systems (CMS) and dynamic, specially coded websites exacerbate the problem. Many sites automatically add tags, allow multiple paths (and URLs) to the same content, and add URL parameters for searches, sorting, currency options, etc. Your website can have thousands of duplicate URLs without you even realizing it.

Choosing the appropriate canonical URL for each set of similar URLs improves the site’s SEO. This is because the browser knows which version canonical is, so any links pointing to different versions can be counted as links to the canonical version. Setting up a canonical URL is similar in concept to a 301 redirect, only without the actual redirect.

How to choose a canonical URL?

When you have a choice between several URLs, canonization is the process of choosing one of them. In many cases, it will be obvious: one URL will be a better choice than others. In some cases, it may not be so obvious, but even then it’s pretty simple: just pick one!

Canonical URLs can be self-referential. It’s a perfectly legitimate option to set the URL to that page as a canonical URL. This can often be a cause of confusion. Also, cross-using a canonical URL between multiple pages can be confusing. For example, setting up a canonical URL for page A, a URL for side B, is not the happiest solution. Also, chaining URLs (e.g. the canonical URL for page B is the URL of page A, and the canonical URL of page C is the URL of page B) is also bad practice. Send clear signals or you’ll force search engines to make bad decisions.

If you’re not sure whether you want to apply a 301 redirect or set a canonical URL, which is a better solution? The answer is simple: you should always do a redirect, unless there are technical reasons not to do so. If you can’t redirect because it would hurt the user experience or be otherwise problematic, set a canonical URL.

Like any other tool used, the canonical URL should be used carefully and following certain guidelines.

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