A canonical tag is a short piece of HTML in a page's head that says "the official address of this content is this URL". When the same page can be reached at several addresses, it tells search engines which one to show in results and credit with links.
How rel=canonical works
The tag looks like <link rel="canonical" href="https://example.com/shoes/trail-runner">. It can also be sent as an HTTP Link header, which is useful for PDFs. Google treats it as a strong hint, not an order. It weighs the tag alongside redirects, sitemap entries, internal links and HTTPS when choosing which duplicate to keep in its index, and it can pick a different URL if the signals disagree. Search Console's URL Inspection shows both the canonical you declared and the one Google selected.
Duplicate addresses appear more easily than most owners expect: with and without www, with a trailing slash, with tracking parameters like ?utm_source=newsletter, sorted product lists, or printable versions.
An example from an online shop
A tea shop sells one green tea that appears in two categories, so it lives at /green-tea/sencha and /gifts/sencha. Ad campaigns add ?gclid= parameters on top. Without guidance, Google may split signals across these versions or pick the gift URL to rank. Putting a canonical tag pointing to /green-tea/sencha on every version, including that page itself, consolidates them so one clean URL appears in search.
Do small sites need canonical tags?
Most modern site builders and CMSs add a self-referencing canonical to each page automatically, and that covers the common cases. Pay closer attention if you sell products in several categories, republish articles on other sites, or run filtered listings. If a page has moved for good, a 301 redirect is a stronger signal than a canonical, because visitors are sent to the right place too.
Canonical tag mistakes
- Every page pointing to the homepage, often from a template bug. Google may then ignore your inner pages.
- Relative or wrong-protocol URLs. Use the full absolute https address.
- Canonical plus noindex on the same page, which gives Google contradictory instructions.
- Pointing pagination page 2 to page 1, which hides the items listed on later pages.
- A canonical URL that redirects or returns 404. The target must be a live 200 page.
Related terms
- 301 redirect: the firmer way to merge duplicate URLs.
- URL: the address variations canonicals resolve.
- XML sitemap: should list only canonical URLs.
- Technical SEO: the wider set of signals canonicals belong to.