Canonical tags for SEO: which address search keeps

A canonical tag is a <link rel="canonical"> element in a page's <head> that names the main address among several showing the same content. Google calls it "a strong signal that the specified URL should become canonical": a signal, not a command. Among the meta tags a page carries, it is the one that speaks for other addresses.

Where copies of one page come from

Copies of one page come from addresses, not from content: the same page is reachable at more than one URL, and each URL looks like a separate page until something says otherwise. On example.com, all of these can return the same product:

• https://example.com/atlas-40/
• https://example.com/atlas-40 (no trailing slash)
• http://example.com/atlas-40/ and https://www.example.com/atlas-40/
• https://example.com/atlas-40/?utm_source=newsletter
• https://example.com/atlas-40/?colour=green
• https://example.com/print/atlas-40/

Nobody built six pages. A server that answers on both protocols, a tracking parameter, a filter and a print template did it between them. Search sees six addresses and has to decide which one to show.

What the tag says, and how strongly

The tag states a preference and leaves the choice to the engine. Google's documentation lists the methods in order of how strongly they influence the choice:

• Redirects: "A strong signal that the target of the redirect should become canonical."
• rel="canonical" link annotations: "A strong signal that the specified URL should become canonical."
• Sitemap inclusion: "A weak signal that helps the URLs that are included in a sitemap become canonical."

The same page adds that none of these is required: if you state no preference, Google "will identify which version of the URL is objectively the best version to show to users in Search." The tag matters when you know better than the engine which address should represent the page, and you usually do. That documentation was last updated on 2026-07-10 and checked on 2026-10-09.

How to write the tag

The tag goes in the <head> and carries one absolute address:

<link rel="canonical" href="https://example.com/atlas-40/">

• Use the full address

  Google supports relative paths but does not recommend them, because they "can cause problems in the long run", for example when a test copy of the site gets crawled and every page on it points at itself.

• Put it on the main page too

  Google recommends a canonical on the canonical page itself, a self-referential canonical. It costs one line and removes any doubt when parameters are added to that address from outside.

• Say the same thing everywhere

  Pick one method per page, or make all of them agree. A sitemap listing one address while the tag names another is two contradicting statements, and Google's list of best practices names exactly that case.

• Use the header for files

  A PDF has no <head>. For non-HTML files, the same statement goes in an HTTP Link header with rel="canonical".

Mistakes Google names

Google's documentation names a short list of things not to do, and each is a way of saying something other than what was meant.

• A fragment as canonical. Google generally does not support URL fragments, so an address ending in #section names nothing it can use.
• Different canonicals by different methods. The tag says one address, the sitemap another; the signals cancel rather than add up.
• Blocking or removing instead of declaring. Google advises against using robots.txt, noindex or the URL removal tool to pick a canonical within a site, because they hide pages rather than point between them. Those directives are a subject of their own and are not covered here.

A further, quieter mistake is pointing the tag at an address that does not return the page: a redirect, an error or a different page entirely. The tag then names something that cannot be the main version, and the engine has to fall back on its own choice.

Canonical tag or redirect

A redirect is for an address that should stop existing; a canonical tag is for an address that should keep working. When an old URL has moved, a visitor and a crawler should both land on the new one, and a redirect does that while also being the strongest signal on Google's list.

When the copy has to stay reachable, as with a filtered view, a tracking link or a print version, a redirect would break the thing people use. The canonical tag lets the address keep serving visitors while asking search to credit the main one.

Canonical and hreflang together

Language versions are not duplicates of each other, and the canonical tag should not merge them. Google's guidance is to "specify a canonical page in the same language, or the best possible substitute language if a canonical page doesn't exist for the same language." Each language version names itself, and the hreflang set names all of them. How the two tags work side by side, and how the combination breaks, is covered under hreflang and canonical.

Checking what is declared

The check reads the delivered page rather than the template. Open the page source of each address variant and confirm that every one names the same absolute URL, that the URL it names returns the page with a success status, and that the sitemap lists that same URL. Copies of a page often carry copies of its title as well; finding those across a site is covered under duplicated titles and descriptions.

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