Definition
A canonical tag is a link element naming the preferred URL when the same content lives at more than one address. It is written as rel="canonical" and was defined in RFC 6596 in April 2012. Google's own wording is the part to hold onto: indicating a canonical preference is a hint, not a rule.
Example. /shoes?utm_source=newsletter and /shoes show the same page. A canonical tag on both, pointing at /shoes, asks Google to treat that one URL as the version to show.
You can see the canonical any page declares, and whether it points somewhere else, with the free Noindex Checker.
Is a canonical tag a directive?
A canonical tag is not a directive. Google calls it a hint, not a rule, and says it may choose a different page as canonical than you do. That is the single most misunderstood fact about the tag: adding it states a preference, it does not settle the question.
You can see when Google disagrees with you. The Search Console page indexing report has a status for exactly that case, "Duplicate, Google chose different canonical than user", and it is the first place to look when the URL you nominated is not the one appearing in results.
How strong is a canonical tag compared with other signals?
A canonical tag is one of the two strong signals, ranked just behind a redirect. Google lists the methods in order of how strongly they can influence its choice, and notes that they stack, so signals pointing the same way reinforce each other.
| Method | Google's description of its strength |
|---|---|
| Redirect | A strong signal that the redirect target should become canonical |
rel="canonical" link | A strong signal that the specified URL should become canonical |
| Sitemap inclusion | A weak signal that helps the listed URLs become canonical |
Google also weighs whether a page is served over HTTPS. The practical rule follows from the table: when your signals disagree, for example a sitemap listing one URL while the tag names another, you have handed the decision back to Google.
What should you never use to pick a canonical?
Never use robots.txt or noindex to choose between duplicates. Google says not to use robots.txt for canonicalisation, because a disallowed URL can still be indexed without its content. It also advises against noindex for this job, because noindex blocks the page from Search completely rather than pointing at a preferred version.
Is duplicate content a penalty?
Duplicate content is not a penalty. Google states that some duplicate content on a site is normal and is not a violation of its spam policies. The real cost is split signals and Google picking a version you did not want, which is the problem a canonical tag exists to solve.
The tag can also point across domains. RFC 6596 allows the preferred version to sit on a different hostname, which is how syndicated copies point back at the original. Check which URL Google actually chose rather than assuming it followed you.
Sources
- RFC 6596: The Canonical Link Relation (April 2012)
- Google Search Central: What is canonicalization
- Google Search Central: How to specify a canonical URL
- Search Console Help: Page indexing report
Last checked against these sources on 19 October 2026.