What Are Canonical Tags and Why They Are Important for SEO

What exactly is a canonical tag?

A canonical tag (also known as "rel canonical") informs search engines that a specific URL represents the master copy of a page. The use of the canonical tag avoids issues caused by identical or "duplicate" content appearing on multiple URLs. In practise, the canonical tag instructs search engines as to which version of a URL you want to appear in search results.

What is the significance of canonicalization?

Duplicate content is a complicated topic, but when search engines crawl a large number of URLs with identical (or very similar) content, it can cause a number of SEO issues. For starters, if search crawlers have to sift through too much duplicate content, they may overlook some of your unique content. Second, large-scale duplication may dilute your ability to rank. Finally, even if you’re content ranks, search engines may choose the incorrect URL as the "original." Canonicalization allows you to control duplicate content.

Best practices for canonical tags

Duplicate content issues can be extremely tricky, but here are a few things to keep in mind when employing the canonical tag:

Self-referential canonical tags

It is all right for a canonical tag to top to the present URL. In other terms, if URLs X, Y, and Z are duplicates and X is the canonical version, putting the tag pointing to X on URL X is suitable. This may appear obvious, but it is a common source of confusion.

Canonicalize your home page ahead of time.

Given the prevalence of homepage duplicates and the fact that people may link to your homepage in a variety of ways (over which you have no control), it's usually a good idea to include a canonical tag in your homepage template to avoid unforeseen problems.

Check your dynamic canonical tags for errors.

When a site has bad code, it may generate a different canonical tag for each version of the URL, completely missing the point of the canonical tag. Check your URLs carefully, especially on e-commerce and CMS-driven sites.

Avoid sending mixed signals.

If you send mixed signals, search engines may ignore a canonical tag or interpret it incorrectly. To put it another way, don't canonicalize page A->page B and then page B->page A. Don't, for example, canonicalize page A to page B and then 301 redirect page B to page A. It's also not a good idea to chain canonical tags (A—>B, B—>C, and C—>D) if possible. Send clear signals, or search engines will make poor decisions.

Be cautious when canonicalizing near-duplicates.

Most people associate canonicalization with exact duplicates. On near-duplicates (pages with very similar content), the canonical tag can be used, but proceed with caution. There is some disagreement about this, but it is generally acceptable to use canonical tags for very similar pages, such as a product page that only differs by currency, location, or a minor product attribute. Remember that non-canonical versions of that page may not be ranked, and if the pages are too dissimilar, search engines may ignore the tag.

Cross-domain duplicates should be canonicalized.

If you own both sites, you can use the canonical tag across domains. Assume you own a publishing company that publishes the same article on a half-dozen different websites on a regular basis. Using the canonical tag will focus your SEO efforts on a single website. Keep in mind that canonicalization will prevent non-canonical sites from ranking, so make sure this is the best option for your business.

301 redirects vs. canonical tags

One frequently asked SEO question is whether canonical tags, like 301 redirects, pass link equity (PageRank, Authority, etc.). They appear to in most cases, but this can be a dangerous question. Keep in mind that these two approaches produce very different outcomes for search crawlers and site visitors.

Human visitors will be automatically directed to Page B if you 301 redirect Page A—>Page B, and will never see Page A.If you rel-canonicalize Page A—>Page B, search engines will know that Page B is the canonical version, but users will be able to access both URLs. Check that your solution corresponds to the desired outcome.

How to Perform an SEO Canonical Tag Audit

There are several things to look for when auditing your canonical tags for optimal SEO performance. Here's a list to get you started:

Is there a canonical tag on the page?

Is the canonical pointing to the correct page?

Are the pages indexable and crawlable?

A common mistake is to point the canonical to a URL that is either blocked by robots.txt or set to "noindex." Search engines may receive mixed and confusing signals as a result of this.