Canonical URL

A canonical URL is the single version of a page that you designate as the primary one when the same or very similar content is reachable at multiple URLs. You declare it with a rel="canonical" tag in the page's HTML.

Duplicate URLs happen constantly without anyone intending them. The same product page might be reachable at example.com/service, example.com/service?ref=email, and www.example.com/service. To a person these look identical; to Google they are three separate URLs with the same content. A canonical tag says "these are all the same page, and this one is the real one."

Without a clear canonical, Google has to guess which version to index and rank, and it may split ranking signals across the duplicates — weakening all of them. Setting a canonical consolidates that authority onto one URL.

Two important honesty points:

• A canonical is a hint, not a directive. Google usually respects it, but it can choose a different canonical if its own signals disagree — for example if your internal links and sitemap point elsewhere.
• Consistency is everything. Your canonical tag, internal links, sitemap, and redirects should all point at the same preferred URL. Mixed signals confuse Google and can leave the wrong version indexed.

Canonical mistakes are a surprisingly common reason pages get crawled but not indexed. If you are cleaning up duplicate URLs, our technical SEO basics guide covers how the pieces fit together.