Canonical tag

A canonical tag (rel="canonical") declares the preferred URL for a piece of content. When the same or very similar content is reachable at multiple URLs, through tracking parameters, session IDs, http and https, trailing slashes, pagination, or print versions, the canonical tells search engines which one to index and consolidate ranking signals onto.

Getting canonicalization right prevents two problems: diluting ranking signals across duplicate URLs, and wasting crawl budget on near-identical pages. The canonical is a strong hint, not a directive; Google can choose a different canonical if your signals conflict, for example if internal links, sitemaps, and redirects all point somewhere other than your declared canonical. Consistency across all of those signals is what makes the canonical stick.

Common mistakes are pointing every page's canonical at the homepage (which can deindex the rest of the site), canonicalizing paginated pages to page one (which hides deeper content), and canonical chains where A points to B which points to C. The rule is that each canonical should point to a single, self-referencing, indexable URL that actually serves the content. We check canonical presence, self-reference, and conflicts against sitemaps and redirects.