Canonical URLs: Preventing Duplicate Content
Canonical tags tell search engines which version of a page is the real one. Get them wrong and you split your own authority.
What is a canonical URL?
A canonical URL is the version of a page that you want search engines to treat as the original. You declare it with a <link rel="canonical"> tag in the HTML head.
When multiple URLs serve the same or similar content, the canonical tells search engines which one to index and which ones to ignore.
Why it matters
Without canonicals, search engines have to guess which version of a page is the real one. They might pick the wrong one, or worse, treat each version as a separate page — diluting your authority across all of them.
Common mistakes
Self-referencing canonicals are correct
Every page should have a canonical that points to itself. This is not redundant — it is a declaration: this URL is the real one. Without it, a query-parameter variant (e.g. ?utm_source=twitter) becomes a separate page in the index.
Canonical to a 404
A page whose canonical points to a URL that returns 404. Search engines see a page claiming its real version does not exist.
Canonical chain
Page A canonicalizes to Page B, which canonicalizes to Page C. Search engines may follow one hop but are not required to follow two.
HTTP vs HTTPS mismatch
A canonical that points to http:// when the site serves https://. This splits the signal between two origins.
Related: What Is Technical SEO and Why It Matters covers the full landscape these canonicals sit within.