A canonical URL is a way for search engines to indicate that a particular URL is a master copy of a page. The use of a canonical tag prevents problems caused by identical or "duplicate" content appearing at multiple URLs. Practically speaking, the 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. You can, for example, have a post or product that is tied to two categories and exists under two URLs.

Canonical URL and duplicate content

Duplicate content is an important topic for SEO, and therefore an important item when it comes to website design and development. When search engines search many 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 some unique content. Second, the existence of duplicate content may reduce the possibility of ranking. Even if the content is ranked, 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, they can be quite large. Modern content management systems (CMS) and dynamic, specially coded sites further exacerbate the problem. Many sites automatically add tags, allow multiple paths (and URLs) to the same content, and add URL parameters for search, sorting, currency options, and more. A site can have thousands of duplicate URLs without you even being aware of it.

Selecting the appropriate canonical URL for each set of similar URLs improves the SEO of the site. This is because the browser knows which version canonical, so all links pointing to different versions can count as links to the canonical version. Setting a canonical URL is similar in concept to redirection 301, only without actual redirection.

How to choose a canonical URL?

When you have a choice between several URLs, canonization is the process of selecting one of them. In many cases, it will be obvious: one URL will be a better choice than another. In some cases, it may not be so obvious, but even then it is quite simple: just choose one!

Canonical URLs can be self-referential. It is quite a legitimate option to like canonical URL set the URL to that page. This can often be a cause for confusion. Likewise, cross-using a canonical URL between multiple pages can be confusing. For example, setting for canonical The URL for page A, the URL for page B, is not the happiest solution. Likewise, chain linking URLs (e.g., the canonical URL for page B is the URl of page A, and the canonical URL for page C is the URL of page B) is also a bad practice. Send clear signals or you will force search engines to make bad decisions.

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

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

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