Canonical URL

A canonical URL is the preferred version of a webpage that you designate as the authoritative source when the same or substantially similar content is accessible at multiple URLs. You declare it by placing a rel="canonical" link tag in the HTML head of each page, pointing to the URL that search engines should treat as the primary version. This prevents duplicate content issues where Google and other search engines might see multiple URLs with the same content and either split ranking signals between them or choose the wrong version to display in search results.

Duplicate content situations are more common than most site owners realize. The same page might be accessible with and without the www prefix, over both HTTP and HTTPS, with trailing slashes and without, with tracking parameters appended by marketing campaigns, through paginated series where content overlaps, or via print-friendly versions and AMP pages. Without canonical tags, each of these URLs appears to search engines as a separate page competing with itself. The canonical tag consolidates the ranking signals from all duplicate versions onto the single URL you designate, ensuring that link equity, click data, and indexing priority are concentrated rather than diluted.

Correct implementation requires that the canonical URL is self-referencing on the primary page, meaning the preferred URL points to itself, and that all duplicate or alternative versions point to that same preferred URL. Common mistakes include setting canonicals to relative URLs instead of absolute URLs, pointing canonical tags to pages that return 404 errors or redirects, and using canonical tags when a 301 redirect would be more appropriate. When content truly exists at multiple URLs and you want to keep all versions accessible to users, the canonical tag is the right choice. When the duplicate URLs serve no user purpose, a 301 redirect is cleaner because it sends both users and search engines to the correct location without relying on a hint that search engines may choose to ignore.