What is a Canonical Tag?

This is a method of communicating to search engines that a specific URL represents the master copy of a page. A canonical tag prevents problems that are caused by identical or duplicate content appearing on numerous URLs. The canonical tag is used to tell the search engine the version of the URL that someone wants to appear in the search results.

How is a Canonical Tag Used?

Duplicate content is complicated, but when search engines have many URLs with similar content, various SEO problems arise. When the search crawlers sift through many similar contents, they can miss the user's unique content. Large scale duplication may also reduce the ranking ability of an individual. Another problem is that even if the one's content does not rank, the search engines may identify the wrong URL as the original. Canonization helps to minimize duplicate content.

The problem with URLs

Most people think of a page as a concept. For search engines, each unique URL represents a separate page. For example, search engines can reach a homepage in each of the following ways.

• http://www.example.com
• https://www.example.com
• http://example.com
• http://example.com/index.php
• http://example.com/index.php?r...

To a person, all these URLs lead to the same page, but to a search engine, each URL is a unique page. This is, however, a small sample of the variations that one might encounter. Modern content management systems and code-driven websites which are dynamic worsen the problem even more. Most sites add tags automatically, allow multiple URLs to the same content and add URL parameters for searches, currency options, sorts, etc.

Canonical tag best practices

Some of the essential things to be taken into consideration when using canonical tags are as follows:

• Canonical tags can be self-referential. It is alright if a canonical tag points to the current URL. If URLs X, Y, and Z are similar and X is the canonical version, it is okay to put the tag pointing to X on URL X.
• Proactively canonicalize your home-page. Since homepage duplicates are popular and people; thus people may link to someone's homepage in numerous ways, it is advisable for one to put a canonical tag on their homepage template to prevent any problems.
• Spot-check your dynamic canonical tags. One should spot-check their URLs since a bad code may cause a site to write a separate canonical tag for every version of the URL.
• Avoid mixed signals. If one sends mixed signals, the search engines may avoid a canonical tag or misinterpret it. It is also not advisable to chain canonical tags. One should send clear signals to prevent forcing the search engines to make bad choices.
• Be careful canonicalizing near-duplicates. One can canonicalize near-duplicates, but they should do so with caution. Usually, it is okay to use canonical tags for identical pages. However, the non-canonical versions of the page may not qualify for ranking, and if the pages are very different, the search engine may decide to ignore the tag.
• Canonicalize cross-domain duplicates. If one controls both sites, they may utilize the tag across various domains. For example in the case of a publishing company that publishes the same article on numerous websites, using the canonical tag will focus their ranking on just a single site. This is because canonicalization will block the non-canonical sites from ranking.