Canonical URL
The one URL you want search engines to treat as the main version of a page.
A canonical URL is the address you mark as the preferred version of a page when the same content is reachable at several URLs, for example with and without tracking parameters, or with and without a trailing slash. You declare it with a rel="canonical" link in the page's head.
Google treats a rel="canonical" link as a strong signal, not a command: it still chooses the canonical itself. Redirects are also a strong signal, and listing a URL in a sitemap is a weak one. Every page should normally name itself as canonical.
Key characteristics
- Marks the preferred URL among duplicates of the same content.
- Declared with <link rel="canonical"> in the page head, or an HTTP header for files such as PDFs.
- A strong signal to Google, which still makes the final choice.
- Each page should usually point to itself.
Example
A product page is reachable at /shoes?utm_source=email and /shoes. Both carry a canonical link to /shoes, so search engines consolidate signals on that one address.
Frequently asked questions
Is a canonical tag a directive?
No. Google calls rel="canonical" a strong signal. It usually follows it, but it can choose a different canonical if other signals disagree.
Should every page have a canonical URL?
Yes, normally a self-referencing one. Google's site move guidance says each new URL should name itself as canonical.