SEO glossary

Canonical tag

A canonical tag is a link element, rel="canonical", placed in the head of a page to tell search engines which URL is the preferred version when the same or very similar content is available at several URLs. Google treats it as a strong signal rather than a command, and can choose a different canonical.

Reviewed by Gareth Hoyle, Managing Director
Last reviewed 31 October 2026

When was the canonical tag introduced?

Google announced support for rel="canonical" on 12 February 2009, as a way to deal with duplicate URLs created by sort parameters, tracking codes and session IDs. The announcement said link popularity, PageRank and related signals would be consolidated on the preferred URL. Asked whether it was a hint or a command, Google engineer Joachim Kupke called it a hint that Google honours strongly.

How does Google choose a canonical?

Google's documentation lists three ways to state a preference: redirects and rel="canonical" annotations are strong signals, and inclusion in a sitemap is a weak one. None is required, and without them Google picks the version it thinks is best. The signals work best when they agree, with internal links, sitemaps and canonical tags all pointing at the same URL.

Google advises against using robots.txt or noindex to choose a canonical within a site. The URL Inspection tool in Search Console shows both the canonical a page declares and the one Google selected.

Common canonical tag mistakes

Using relative URLs, which Google supports but does not recommend
Pointing every page in a paginated series to page one, when each page should have its own canonical
Letting JavaScript change the canonical set in the HTML source
Declaring different canonicals for the same page through different methods

Example

A product at /trainers/air-runner can also be reached at /trainers/air-runner?colour=blue and /sale/air-runner. Each version carries <link rel="canonical" href="https://www.example.com/trainers/air-runner">, so the links and other signals that point at the variants are consolidated on the one URL that should appear in results.