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Canonical Tag

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Canonical tag is an HTML element that marks which version of a duplicate or near-duplicate page a search engine should treat as the primary one to index, when several page versions contain the same or very similar content.

The tag is written as <link rel=”canonical” href=”URL” /> and placed in the head section of a page, pointing to the URL that should be treated as the authoritative version. Duplicate content can appear for reasons that have nothing to do with copying, such as a product page reachable through several category paths, or a site available with and without a www prefix.

A canonical tag consolidates the ranking signals from these variations toward one chosen URL, rather than splitting them across several near-identical pages. Search engines treat a canonical tag as a strong signal rather than a strict directive, meaning a different URL can still be chosen for indexing if the search engine finds stronger evidence elsewhere.

Example

An online store might list the same t-shirt under both a color filter URL and a size filter URL, producing two pages with nearly identical content. Adding a canonical tag to both pages that points to the main product URL tells search engines to consolidate ranking signals there, rather than treating the filtered versions as separate, competing pages.

Key characteristics

Signal, not a directive: Search engines generally respect a canonical tag but can override it if other signals point to a different preferred URL.
Self-referencing by default: Most pages canonicalize to themselves, which confirms to search engines that a given URL is already the preferred version.
Consolidates ranking signals: Links and authority pointing to duplicate URLs are credited to the canonical URL instead of being split apart.
Placed in the page head: The tag sits in the HTML head section and is not visible to a reader browsing the page.

Related terms

SEO – the broader practice of improving a page so it ranks and performs well in search results, which canonical tags support by resolving duplicate content.
On-page SEO – the category of optimization controlled directly on a page, which technical elements like a canonical tag fall under.
Sitemap – a file listing preferred page URLs, which works alongside canonical tags to help search engines settle on one version of each page.
Robots.txt – a separate technical file that controls crawler access, distinct from a canonical tag, which addresses which version of a page to index.

Frequently asked questions

Does a canonical tag remove duplicate pages from a site?

No, a canonical tag does not delete or redirect duplicate pages; it only signals to search engines which version should be indexed and ranked.

What happens if a page has no canonical tag?

Search engines will usually choose a version to index on their own using other signals, which can sometimes select a URL the site owner would not have preferred.

Can a canonical tag point to a page on a different domain?

Yes, a canonical tag can point to a URL on another domain, which is sometimes used for syndicated content republished across multiple sites.

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FAQ

What is a canonical tag?

A canonical tag is an HTML element that marks which version of a duplicate or near-duplicate page a search engine should treat as the primary one. It is written as a link tag with rel equal to canonical, placed in the head section of a page. Search engines use it to consolidate ranking signals from several similar URLs into 1 preferred URL. Most content management systems can generate this tag automatically for every page.

Does a canonical tag remove duplicate pages?

A canonical tag does not delete, redirect, or hide duplicate pages, since all versions of the page remain live and accessible to visitors. It only signals to search engines which version should be indexed and credited with ranking signals. A site can have 5 or more near-duplicate URLs for a single product without a canonical tag causing any of them to disappear. Removing duplicate content entirely usually requires a 301 redirect instead.

What happens without a canonical tag?

If a page has no canonical tag, search engines generally choose which version to index using other signals, such as internal linking patterns or a sitemap listing. This automatic selection does not always match what a site owner would have preferred, especially across large sites with thousands of similar URLs. Adding an explicit canonical tag removes this guesswork for the search engine. Most SEO plugins add a self-referencing canonical tag by default within seconds of activation.

Can a canonical tag point to a different domain?

A canonical tag can point to a URL on a different domain, and this is sometimes used when the same article or content is syndicated and republished across 2 or more websites. Search engines generally treat the tag the same way regardless of whether it points within the same domain or to an external one. This cross-domain use helps avoid duplicate content penalties when content is intentionally shared. It requires cooperation from both sites involved, since only 1 of them typically holds the canonical version.

Is a canonical tag always followed by search engines?

Yes, a canonical tag is considered a signal search engines generally respect rather than a strict directive they must always follow. Search engines can choose a different URL as canonical if other signals, such as internal links or a sitemap, point strongly elsewhere. This override happens on a minority of pages, though the exact rate varies by site. Setting a clear, self-referencing canonical tag on every page reduces how often this kind of override occurs.