Canonical tag

When to use it

• Multiple URLs serving same content: tracking parameters (?utm_source=…), session IDs, sort/filter combinations, mobile vs desktop subdomains
• Pagination: canonicalize each paginated page to itself (self-canonical), not to page 1
• Syndicated content: cross-domain canonical to credit the original publisher

When NOT to use it

• Different content across pages — that’s not duplication, that’s separate pages
• Across language variants — use hreflang for that, not canonical (the two solve opposite problems)
• To “merge” two competing pages into one — that’s a redirect (301), not a canonical

Common implementation mistakes

• Canonical pointing to a redirect target (Google follows but it adds a hop — point directly to final URL)
• Canonical pointing to a different language (breaks hreflang signals)
• Inconsistent canonicals across paginated sets
• Missing self-canonical on the canonical URL itself
• Conflicting signals: rel=canonical + hreflang + 301 + meta robots noindex all pointing different directions

Validation

• Google Search Console → URL Inspection → “Google-selected canonical” vs “User-declared canonical”
• Tools: Screaming Frog (canonical column), Sitebulb, Ahrefs Site Audit

If Google’s selected canonical disagrees with yours, that’s a signal something’s off — usually weak signal strength or conflicting directives.