What Is a Canonical URL?
A canonical URL is the "official" version of a page. When the same content exists at multiple URLs — example.com/page, example.com/page?ref=newsletter, and www.example.com/page — the canonical tag tells search engines which one to treat as the original.
Why Duplicate Content Is a Problem
Search engines want to show one best result for each query. If your content appears at several URLs, Google must guess which to rank — and it may pick the wrong one, or split your ranking signals across all of them. Either way, you lose.
How the Canonical Tag Works
In the page's HTML, you add a line telling search engines the real address:
<link rel="canonical" href="https://www.example.com/page">All the duplicate versions point to this one canonical URL, so your authority stays concentrated.
When Canonicals Matter Most
URL parameters (tracking, sorting, filters)
WWW vs non-WWW versions
HTTP vs HTTPS versions
Syndicated or republished content
Print-friendly versions of pages
Common Mistakes
Pointing the canonical to a page that redirects
Using different canonicals for desktop and mobile
Canonicalizing every page to the homepage
The Bottom Line
A canonical URL is a simple line of code that prevents a surprisingly expensive SEO problem. Most platforms set it automatically — but if you manage content across many URLs, it's worth understanding.