What Is a Canonical Tag and When to Use It

The short answer

A canonical URL is the main version of a page — the one you want to appear in search. When the same content is available at several addresses, the rel="canonical" tag tells search engines: "index and rank this address, the others are copies of it".

The tag goes in the <head>:

<link rel="canonical" href="https://example.com/catalog/shoes/" />

Important: canonical is a hint, not a directive. Google and Yandex weigh it together with other signals (redirects, internal links, sitemap) and may pick a different version if those signals conflict.

Where duplicates come from

Sites often create duplicates on their own:

• URL parameters: ?utm_source=..., ?ref=..., session IDs;
• sorting and view options: ?sort=price, ?view=grid;
• www and non-www, http and https, trailing slash or not;
• one product in several categories: /men/sneakers/model-x and /sale/model-x;
• print versions, AMP pages, content republished on other sites.

Without a canonical, the search engine decides on its own which version is primary, and link equity and behavioural signals are split between the copies.

When to use canonical

A self-referencing canonical points to the page itself. Put it everywhere: it protects you from parameter duplicates you did not anticipate.

When canonical is the wrong tool

• Pages with different content. If a "red sneakers" filter is a real landing page targeting its own query, do not fold it into the general catalog.
• Pagination. Pointing page 2 and beyond to page 1 is a common mistake: products on deeper pages lose their path into the index. Usually each paginated page needs its own canonical.
• A permanent move. If the old address is no longer needed, use a 301 redirect, not a canonical.
• Keeping a page out of the index. That is what noindex is for; canonical does not guarantee exclusion.

Mistakes that get pages deindexed

• Every page points to the home page. Often caused by a CMS template with a hard-coded canonical. The search engine may treat the whole site as a duplicate of the home page.
• Canonical to a page that redirects, returns 404 or is noindex. The signals conflict, and the engine ignores the hint or drops both versions.
• Relative or wrong addresses. A wrong protocol, domain or a stray parameter in the canonical causes confusion. Use absolute URLs.
• Several canonicals on one page. For example, one from the theme and one from an SEO plugin. Search engines may then ignore both.
• A JavaScript-injected canonical that differs from the HTML. Put it in the source HTML.
• Chains. Page A points to B, and B points to C. Point straight to the final address.

How to check

1. Open the page source and find rel="canonical" — there should be exactly one, with an absolute URL.
2. In Google Search Console, use URL Inspection: it shows the canonical you declared and the one Google selected.
3. In Yandex Webmaster, check the pages-in-search report: excluded non-canonical pages are listed separately.
4. Crawl the site and look for canonicals that lead to redirects, 404s or the wrong template.