Canonical URL

What it means

A canonical URL is the address a website marks as the main version of a page when the same or very similar content exists at several addresses. The site tells search engines with a tag in the page code, and they then show that address in results and credit its ranking to it.

Duplicate addresses arise more often than it seems: with and without “www”, with and without a trailing slash, sorting and filter parameters in an online shop, UTM parameters from campaigns, or print versions. To a person it is one page; to a search engine it is several.

The canonical tag is a hint, not an order. Search engines usually respect it if it is consistent with other signals: internal links, the XML sitemap and redirects should all point to the same address.

How it works in practice

In a technical site audit we check that every important page carries a self-referencing canonical tag and that duplicate variants point to the main version. On multilingual sites consistency with hreflang is checked too: each language version is canonical to itself.

In online shops a typical case is a category page with many filter combinations. A canonical address without filters keeps thousands of near-identical pages out of the index and concentrates ranking on the page the company wants shown.

Where possible, duplicates are better prevented, for example with a permanent redirect from the non-“www” version to the “www” version. The canonical tag is the solution where variants must stay accessible, such as filters or campaign parameters.

What to watch out for

The most common mistake is a template that sets the home page as canonical on every page. Search engines may then drop the whole site from the index except one page. A check after every template change is worthwhile.

The second risk is conflicting signals: the canonical tag points to one address, the sitemap to another and a redirect to a third. The search engine then chooses for itself, and it may not pick the version the company wanted.