GlossarySEO
What is a canonical tag?
Also called: rel=canonical, canonical URL
Definition
A canonical tag is a line in a page's HTML, <link rel="canonical" href="…">, that tells search engines which URL is the main version when several URLs show the same or very similar content.
Canonical tag, explained
Most sites serve the same content at more than one address without meaning to. example.com/pricing, example.com/pricing/, example.com/pricing?ref=twitter and the http:// version can all return the same page. To a search engine those are four URLs. It has to pick one to show in results and fold the others into it. That process is called canonicalization, and the canonical tag is how you state your preference.
Google's documentation lists three main ways to signal a canonical, in order of strength: a redirect (a strong signal), a rel="canonical" link annotation (also a strong signal), and listing the URL in your sitemap (a weak signal). They stack, so using them together is more reliable than any one alone.
The key word is signal. A canonical tag is a strong hint, not a command. If your canonical points to a page that looks different, is blocked, returns an error or is contradicted by your internal links and sitemap, Google can pick another URL. Search Console will then report "Duplicate, Google chose different canonical than user".
A good setup is boring. Every indexable page has a self-referencing canonical with the absolute URL. Internal links, the sitemap and the canonical all point to the same version. Tracking parameters never change the canonical. Pages that are truly separate, like two different products, each canonicalize to themselves, even if they share a template.
On a Next.js site, the cleanest approach is a single metadata helper that builds the canonical from the route path, so no page can forget it or get it wrong. That's what we use on this site and on our own product.
Why it matters for founders
If Google splits your signals across duplicate URLs, the version you care about can rank worse or not appear at all. A correct canonical keeps links, clicks and relevance pointed at one URL. It also stops parameter-heavy pages from wasting crawls on a young site.
Example
Your pricing page gets shared as /pricing?utm_source=newsletter. Its HTML includes <link rel="canonical" href="https://example.com/pricing">, so Google treats the tagged link as the same page and shows the clean URL in results.
Common mistakes
- Pointing every page's canonical at the homepage. Google usually ignores this, and it tells it nothing useful.
- Using relative or mixed URLs (
httpvshttps,wwwvs bare domain) so the canonical disagrees with your redirects. - Canonicalizing a page to a URL that is noindexed, redirected or blocked in robots.txt.
- Letting internal links and the sitemap point to a different version than the canonical tag does.
Sources
Checked
Related terms
- NoindexNoindex is a rule, set in a robots meta tag or an X-Robots-Tag HTTP header, that tells search engines not to show a page in their results even though they can crawl it.
- XML sitemapAn XML sitemap is a file that lists the URLs you want search engines to crawl and index, optionally with the date each one last changed. It helps discovery; it doesn't guarantee indexing.
- 301 vs 302 redirectA 301 redirect says a page has moved permanently, so search engines should treat the new URL as canonical. A 302 says the move is temporary, so they keep showing the original URL.
- IndexingIndexing is when a search engine analyzes a crawled page and stores it in its index, the database it serves results from. A page that isn't indexed can't appear in search results.