Canonical Tag

Definition

Canonical Tag - An HTML element (rel="canonical") that tells search engines which version of a page is the 'official' one when multiple similar or duplicate URLs exist. Prevents duplicate content issues without removing pages.

In more depth

The canonical tag (<link rel="canonical" href="URL">) is placed in a page's <head> to signal to search engines: "this is the preferred URL for this content." It's commonly used when the same content is accessible via multiple URLs - for example, with and without trailing slashes, with different URL parameters, or through HTTP and HTTPS versions.

E-commerce sites are particularly reliant on canonical tags because product pages often exist at multiple URLs (with different filters, sorting parameters, or facet combinations). Without canonicalisation, Google may split ranking authority across dozens of near-identical URLs instead of concentrating it on the one version you want to rank.

Self-referencing canonical tags - where a page points to its own URL as the canonical - are best practice even when there's no duplication risk. They reinforce the preferred URL to Google and prevent external sources from accidentally creating canonical confusion by linking to variations of the URL.

Related terms: Indexation - URL Structure - Crawlability

Frequently asked questions

What's the difference between a canonical tag and a 301 redirect?

A 301 redirect sends users (and search engines) to a new URL permanently - the old URL stops being accessible. A canonical tag keeps both URLs accessible but tells Google which one to index and rank. Use 301 redirects when you want to permanently consolidate two URLs; use canonical tags when you need both URLs to remain accessible.

Can I use canonical tags across different domains?

Yes - cross-domain canonical tags tell Google that content on one domain is the original source of content also appearing on another domain. Publishers and syndication networks use these to prevent their content from being outranked by sites that republished it.