A canonical URL is a URL that search engines use to determine the “master” version of a group of duplicate, or near-duplicate pages. Canonical URLs are mainly used to help solve duplicate content issues.
Pages with duplicate or similar content may confuse search engines, as they wouldn't know which of the 2 (or more) similar pages deserve to rank more.
A canonical link can be placed in the header of the document and follows this format:
<link rel="canonical" href="https://ahrefs.com/blog/seo-metrics/>
More information about why, and how to use canonical URLs can be found in our Glossary article for Canonical URLs.
Checking for Canonical URLs in Ahrefs
In Site Explorer, canonical links are marked with a respective tag:
Backlinks to non-canonical versions of websites will also have their respective canonical URL shown:
The Backlinks report in Site Explorer also provides an option to filter for pages using a Canonical tag
rel="canonical" to refer to your website:
Canonical URLs can also be identified in Site Audit by going to any completed Site Audit, go to Page Explorer, and filter for "Canonicalized":