A canonical URL is the master version of a URL when multiple URLs display the same or very similar content. You declare it using a rel="canonical" link element in the page's <head>. This tells search engines which URL to index and to consolidate ranking signals towards.
HTML Implementation
<link rel="canonical" href="https://www.example.com/blog/seo-guide" />When Duplicate Content Occurs
- HTTP and HTTPS versions of the same page.
- WWW and non-WWW versions.
- URL parameters (e.g.
?sort=price,?ref=email). - Printer-friendly or mobile versions of a page.
- Syndicated content published on multiple domains.
Self-Referencing Canonicals
It is best practice to add a canonical tag to every page pointing to itself, even if there is no duplicate. This prevents accidental indexing of parameter-laden URLs and clarifies the preferred URL to crawlers.
Canonical vs 301 Redirect
A 301 redirect actively sends users and bots to the new URL. A canonical is a hint — Google usually follows it but may ignore it. Use 301 for truly dead URLs; use canonical for parallel versions that must remain accessible.