Canonical URL

Definition

A canonical URL is specified using a <link rel="canonical"> tag in the HTML head. It tells search engines which URL should be treated as the authoritative version of a page when the same content is accessible at multiple URLs. This prevents duplicate content issues that can dilute ranking signals.

Common scenarios where canonical tags are needed: HTTP vs HTTPS versions, www vs non-www, URL parameters (sorting, filtering), print-friendly versions, and content syndicated across multiple domains.

Why It Matters

Without canonical tags, search engines may treat multiple URLs as separate pages competing for the same rankings, splitting link equity and potentially triggering duplicate content penalties. Canonical tags consolidate all ranking signals to a single URL.

Canonical tags are also important when syndicating content. If you publish an article on your site and it gets republished on Medium or a partner site, the canonical tag on the republished version should point back to your original URL to ensure your site gets the ranking credit.

Examples

Your product page is accessible at all of these URLs:

https://example.com/products/widget
https://example.com/products/widget?color=blue
https://example.com/products/widget?ref=newsletter
http://example.com/products/widget

All four should include this canonical tag:

<link rel="canonical" href="https://example.com/products/widget" />

This tells Google to treat the clean HTTPS URL as the official version and consolidate all ranking signals there.