What is a canonical tag?
The same page often lives at several URLs without anyone meaning it to: with and without a trailing slash, http and https, with tracking parameters, sorted and filtered versions of a product list, a print version, a syndicated copy on a partner site. To a search engine each is a separate page with duplicate content, and it has to guess which one to index and which to give credit for links. The canonical tag removes the guess. It sits in the head of the page as a link rel="canonical" pointing at the one URL you want to count, and every duplicate carries the same tag pointing at the same place.
It is a hint, not a command. Google usually honors it, and overrides it when the signals disagree, which is why the rest of the site has to agree with it too.
Why it matters
Why a canonical tag matters
It consolidates. Links, shares and ranking signals that would be split across five variants of a page are gathered onto one, which ranks better than any of the five would alone. It keeps the index clean, so the URL that appears in results is the one you designed, not the version with a tracking string on the end. And it protects against duplicate content being read as a quality problem, which on large sites with faceted navigation or many parameters can suppress whole sections.
For content that is republished elsewhere, a cross-domain canonical pointing back to the original is how you keep the credit.
In practice
How a canonical tag works in practice
- 01
Every indexable page should carry a self-referencing canonical: the page says its own clean URL is the one that counts. Variants (parameters, sort orders, session IDs, mobile or print versions) point at that clean URL. Most content management systems and SEO plugins add this automatically, but the automation needs checking after a migration, a domain change or a move to https, because a sitewide canonical pointing at the wrong domain is a sitewide problem.
- 02
Keep the signals consistent. The canonical URL should be the one in the XML sitemap, the one your internal links use, the one that returns a 200, and the one your redirects resolve to. When those disagree, Google picks its own canonical, and the URL Inspection tool in Search Console will tell you which, which is the quickest way to find out your tag is being ignored.
Watch out
What people get wrong about canonical tags
Using them instead of redirects. A canonical leaves both URLs live; if the old page should be gone, a 301 is the tool. Using them across pages that are not actually duplicates, which Google will ignore and may treat as a signal of carelessness. And pointing every page's canonical at the homepage, usually through a template error, which tells Google the whole site is one page.
Forgetting the paginated series, where each page in a list should canonicalize to itself rather than to page one, or the archive quietly disappears from the index.
