Canonical links

Last updated 17 June 2026 3 min

A canonical tag tells search engines which version of a URL is the "main" one when multiple URLs serve identical or near-identical content. It lives in the <head> of a page:

<link rel="canonical" href="https://example.com/product/blue-mug">

Why duplicates exist in the first place

Even on well-built sites, the same content is often reachable through multiple URLs:

http:// vs https://
www. vs non-www.
Trailing slash vs no trailing slash
URL parameters: ?utm_source=, ?sessionid=, ?sort=price
Faceted navigation in e-commerce (filters, colours, sizes)
Print versions, AMP versions, mobile subdomains
Capitalisation differences (/Page vs /page)

Without canonicals, search engines have to guess which URL to index — and may split ranking signals across multiple variants.

What a canonical tag does

Canonical tags are a hint, not a directive. Google considers the canonical tag as one of several signals (alongside redirects, internal links, sitemap inclusion, hreflang) when choosing the canonical URL. Conflicting signals can cause Google to ignore your tag entirely.

Self-referencing canonicals

Best practice is for every indexable page to include a canonical pointing to its primary version. This:

Resolves any tracking parameter, query string, or session ID variants automatically.
Acts as a safety net against accidental duplication.

Common canonical mistakes

Pointing canonicals to noindex pages. Confuses signals and can lead to neither page being indexed..
Inconsistent protocol or trailing slash between the canonical and the actual URL.
Multiple canonical tags on a single page — Google ignores all of them when this happens.
Canonicalising everything to the homepage. This is treated as a soft 404 — the destination has no relationship to the source content.
Pointing cross-domain canonicals to sites you don't own, or that don't reciprocate.
Mismatch between canonical tags and hreflang. These should reference the same canonical version.

Canonicals vs. redirects vs. noindex

Use a 301 redirect when there's only one valid URL and the old one should be retired.
Use a canonical when you need both URLs to remain accessible (e.g., filtered category pages, parameterised URLs) but only one should be indexed.
Use noindex when the page should be accessible but never indexed.

E-commerce considerations

The faceted navigation inherent in most e-commerce sites creates the most canonical complexity. Common patterns:

Self-canonical for the main category page.
Filter combinations canonical to the parent category if they produce no unique value.

Why canonical tags matter for SEO

As above, canonical tags (rel="canonical") tell search engines which version of a page is the "master" copy when duplicate or near-duplicate content exists across multiple URLs.

They're important for SEO because they:

Consolidate ranking signals — link equity and authority from duplicate URLs are pooled to the canonical version rather than split across them
Prevent duplicate content issues — avoid search engines indexing multiple versions of the same page and diluting visibility
Control which URL appears in search results — you specify the preferred version to be indexed and ranked
Manage common duplication scenarios — URL parameters (filters, tracking, sorting), HTTP/HTTPS or www/non-www variants, print versions, and syndicated content

Without them, search engines pick a canonical URL themselves, which may not be the version you want ranking.

Disclaimer: All information contained herein is for informational purposes only. It is not advice or instructional.