🔍 SEO / GEO

Canonical Tag

canonical tag · rel=canonical · canonical url · canonical link · duplicate content tag

Definition

A canonical tag is the HTML attribute rel=canonical that tells a search engine the preferred URL for a page when several versions of it exist with identical or near-identical content.

Why the canonical tag exists

In e-commerce one product is often reachable at several URLs: through different categories, with
sorting parameters, with filters, with UTM tags. A search engine treats every unique URL as a
separate page. Without a canonical, link equity is scattered across the duplicates instead of
concentrating on one page.

Example: a Nike trainer product page is reachable at all of these addresses:

/catalog/shoes/nike-air-max-270
/catalog/men/nike-air-max-270
/catalog/shoes/nike-air-max-270?sort=popular
/catalog/shoes/nike-air-max-270?utm_source=email

Without a canonical, the engine may index all four as independent pages. With one, it knows the
first is the main version and the rest are variants of it.

Where to place canonicals in e-commerce

Scenario	The problem	The fix
A product in several categories	Several URLs with identical content	Canonical to the main category
Pages with UTM parameters	An endless supply of new pages	Canonical to the parameter-free URL
Pagination (/page/2, /page/3)	Duplicate listings	Canonical to the first page, or a self-canonical
HTTP and HTTPS versions	The whole site duplicated	301 redirect plus a self-canonical on HTTPS
Filters and sorting	Parameter duplicates	Canonical to the clean category URL

Technical details

The canonical goes in the <head> of the HTML document:

<link rel="canonical" href="https://example.com/catalog/shoes/nike-air-max-270" />

Or in the HTTP response header, for PDFs and other non-HTML resources:

Link: <https://example.com/catalog/shoes/nike-air-max-270>; rel="canonical"

Important: if the canonical and robots.txt contradict each other — the page is blocked from
crawling but carries a canonical — the engine cannot read the tag and may index the duplicate
anyway. Make sure canonical pages are open to crawling.

Common mistakes

The canonical points at a redirecting page — the engine sees a chain and may not apply the tag
Different canonicals for the same page in different systems — one URL in the HTML, another in
the sitemap
A canonical pointing at an empty or 404 page — automatically ignored
No canonicals on faceted search pages — every filter combination creates a duplicate

Frequently asked questions

Is a search engine obliged to follow the canonical?

No. rel=canonical is a hint, not a directive, unlike noindex. Google may ignore it when it judges another URL to be the better candidate — one with more inbound links, for example. That is why the canonical needs reinforcing: reference only canonical URLs in the sitemap, and point internal links at the right addresses.

Canonical or a 301 redirect — which should you choose?

If the duplicate page should not be reachable by visitors at all, use a 301 redirect. A canonical leaves the page accessible while asking the engine not to index it separately. For UTM versions and sorting parameters the canonical is preferable: the visitor sees a clean experience and the engine sees the right URL.

Does a page need a canonical pointing at itself?

Yes, it is good practice. A self-referencing canonical tells the engine explicitly that this is already the canonical URL. It protects against duplicates appearing accidentally through session or sorting parameters.