A canonical URL is the preferred version of a web page when multiple URLs have the same or very similar content. The rel="canonical" tag tells search engines which version to index and rank, preventing duplicate content issues.

🔹 When Do You Need Canonical Tags?

Same page accessible via http and https
With and without www
With and without trailing slash (/page vs /page/)
With UTM parameters (?utm_source=email)
With session IDs or tracking parameters
Paginated pages (/page/1, /page/2)
Product pages accessible from multiple categories
Print versions of pages
Mobile versions (m.example.com)

🔹 Implementation Methods

1. HTML <head> tag — Most common. Add to every page version.

2. HTTP Header — For non-HTML files like PDFs.

3. XML Sitemap — Only include canonical versions.

4. 301 Redirect — Strongest signal for duplicate pages.

🔹 Critical Rules

✅ Self-referencing canonicals are recommended on every page
✅ Use absolute URLs (https://example.com/page not /page)
✅ Only ONE canonical per page
✅ Canonical must be indexable (not noindexed or blocked)
✅ Canonical URL must return 200 status
❌ Don't canonicalize to a redirected URL
❌ Don't use canonical to replace pagination
❌ Don't put canonical in <body> — only in <head>

🔹 Common Mistakes

❌ Conflicting canonicals — Multiple canonical tags on the same page

❌ Canonical chain — Page A canonicals to B which canonicals to C

❌ Relative URLs — Use /page instead of https://example.com/page

❌ Wrong protocol — Canonical points to http:// when page is https://

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