Canonical Tags: Avoiding Duplicate Content
A Canonical Tag (link rel=canonical) is an HTML element in the head section of a webpage that signals to search engines the preferred URL version when the same or similar content is accessible under multiple URLs.
Key Takeaways
- ✓Canonical Tags signal Google the preferred URL version
- ✓Self-referencing Canonicals on every page are best practice
- ✓Canonical is a hint not a command — Google can ignore it
- ✓Incorrect Canonicals can remove pages from the index
- ✓Cross-Domain Canonicals are possible but rarely recommended
The canonical tag is one of the most important technical SEO elements for avoiding duplicate content.
How Canonical Tags Work
The tag is placed in the head section of each page and points to the preferred URL. When the same content exists under multiple URLs (with/without www, with/without trailing slash, with URL parameters), the canonical tells Google which version should be indexed.
Best Practices
Self-referencing canonicals: Every page should have a canonical tag that points to itself. This is best practice even when no duplicates exist. Use HTTPS version, be consistent with or without trailing slash, use absolute URLs (not relative).
Common Mistakes
Canonical pointing to a 404 page, canonical pointing to a noindex page, conflicting canonicals in sitemap and HTML, canonical chains (A points to B, B points to C). These errors can cause important pages to disappear from the index.
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— John Mueller, Google Search Advocate