Glossary · Beginner
Canonical URL
A canonical URL is the single address you nominate as the authoritative version of a page when several addresses serve the same content.
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Definition
A canonical URL is the address you declare as the authoritative one for a piece
of content. It is expressed with a link element carrying rel="canonical" in
the document head, and it should be absolute.
Why duplicates appear without you trying
Small sites accumulate duplicate addresses by accident. The same page can be
reachable at http and https, with and without www, with and without a
trailing slash, with and without index.html, and with any number of tracking
parameters appended. Each variation is a separate URL to a crawler.
Redirects and canonicals do different jobs
A redirect removes a duplicate by sending everyone to one address. A canonical leaves the duplicate reachable but names the preferred version. Use redirects for host and format variations you control, and canonicals for parameter and filter variations you cannot remove.
Self-referencing is normal
Every indexable page should declare a canonical pointing at itself. This costs nothing, removes ambiguity, and protects against parameters appended by other sites. A canonical that points somewhere unexpected is a common and expensive mistake, because it asks search engines to ignore the page entirely.
Common failures
- A staging or preview canonical shipped to production.
- Relative canonicals that resolve differently on different paths.
- Filtered or sorted views each declaring themselves canonical.
- A canonical pointing at a page that redirects or returns 404.
- Sitemap entries that disagree with the declared canonical.
Review question
Fetch the deployed page and read the canonical the server actually sends. Does it match the address you want indexed, and does that address return 200 without a redirect?