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Canonical URL

Published: · Updated:

A canonical URL is the address indicated to search engines as the primary, preferred version of a page when the same or very similar content is available under multiple addresses. It is declared with a <link rel="canonical" href="..."> tag in the <head> or an HTTP Link header, per RFC 6596.

Duplicates arise naturally: tracking parameters (?utm_...), trailing-slash variants, http/https, www/non-www, sorting variants in e-commerce. Without canonicalization, ranking signals (including links) are spread across several addresses, and the search engine picks the indexed version itself — not always the right one.

Best practices

  • Every indexable page should have a canonical — ideally self-referencing.
  • The canonical should be an absolute URL, consistent with the sitemap and internal linking.
  • Do not combine canonical with noindex on the same page — these are conflicting signals.
  • Pagination and language variants are not duplicates — do not canonicalize them to a single page.

Canonical is a hint, not a directive

Google treats rel="canonical" as a strong suggestion it may ignore if other signals (internal links, sitemap, redirects) point elsewhere. Consistency of all signals is a precondition for effective canonicalization.

Example

A self-referencing canonical on a product page — it points to itself as the primary version even though the same product may be available under a sorting-parameter URL.

Tag in <head>
HTML 
<link rel="canonical" href="https://example.com/products/mens-t-shirt">
Alternatively — HTTP header
HTTP 
Link: <https://example.com/products/mens-t-shirt>; rel="canonical"

Sources & further reading

Related terms

noindex A directive telling search engine bots not to add a page to the index. It does not block crawling — only indexing. Usually set via meta robots or the X-Robots-Tag header.Crawl Budget The number of URLs a search engine bot can and wants to crawl on a site within a given time. Critical for large and frequently changing websites.XML Sitemap An XML file listing a site's URLs with metadata (lastmod) that helps search engines discover and refresh pages. Defined by the sitemaps.org standard.Structured Data A standardized format for describing page content (usually JSON-LD with the Schema.org vocabulary) that helps search engines understand a page and qualify it for rich results.HSTS (HTTP Strict Transport Security) An HTTP header forcing the browser to connect to a site exclusively over HTTPS for a declared period. Protects against SSL stripping attacks.
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